diff --git a/docs/python.md b/docs/python.md
index 29a43fe0..63b9d712 100644
--- a/docs/python.md
+++ b/docs/python.md
@@ -267,7 +267,10 @@ their documents.
 none: `search` returns no results, and `ask` and `analyze` run with no evidence
 from any database.
 
-On the constructor it means something else. Passing `sources` alongside a
+A name no client covers raises `UnknownDatabaseError`, a `KeyError`, wherever it
+is given: at construction, per query, and when placing a citation.
+
+On the constructor `sources=[]` means something else. Passing `sources` alongside a
 database path raises `AmbiguousDatabaseError` immediately, since both say which
 database to open. Passing `sources=[]` alone raises `ValueError` on entering the
 client: a selection of nothing to search is a legitimate question, a client over
diff --git a/haiku_rag_slim/haiku/rag/cli.py b/haiku_rag_slim/haiku/rag/cli.py
index afea9c29..acf0dcbc 100644
--- a/haiku_rag_slim/haiku/rag/cli.py
+++ b/haiku_rag_slim/haiku/rag/cli.py
@@ -27,6 +27,7 @@ from haiku.rag.store.exceptions import (  # noqa: E402
     MigrationRequiredError,
     ReadOnlyError,
     SourceUnavailableError,
+    UnknownDatabaseError,
 )
 from haiku.rag.store.models.chunk import SearchType  # noqa: E402
 from haiku.rag.utils import is_up_to_date  # noqa: E402
@@ -51,6 +52,7 @@ def cli():
         ConfigMismatchError,
         MigrationRequiredError,
         ReadOnlyError,
+        UnknownDatabaseError,
         SourceUnavailableError,
     ) as e:
         typer.echo(f"Error: {e}", err=True)
diff --git a/haiku_rag_slim/haiku/rag/client/__init__.py b/haiku_rag_slim/haiku/rag/client/__init__.py
index db77e016..c13a6e60 100644
--- a/haiku_rag_slim/haiku/rag/client/__init__.py
+++ b/haiku_rag_slim/haiku/rag/client/__init__.py
@@ -34,6 +34,7 @@ from haiku.rag.store.exceptions import (
     MigrationRequiredError,
     ReadOnlyError,
     SourceUnavailableError,
+    UnknownDatabaseError,
 )
 from haiku.rag.store.models.chunk import Chunk, SearchResult, SearchType
 from haiku.rag.store.models.document import Document
@@ -208,9 +209,9 @@ class HaikuRAG:
 
         None only when a client covering a set is given no name, as for evidence
         recorded before databases could be named. A name this client does not
-        cover raises `KeyError`, decided by `clients_covering` so that one
-        database answers a wrong name the same way a set does: provenance naming
-        another database is wrong rather than absent.
+        cover raises `UnknownDatabaseError`, decided by `clients_covering` so
+        that one database answers a wrong name the same way a set does:
+        provenance naming another database is wrong rather than absent.
         """
         if source is None:
             return None if self.covers_multiple else self
@@ -839,7 +840,7 @@ class HaikuRAG:
         covered = set(self.source_names)
         unknown = [name for name in sources if name not in covered]
         if unknown:
-            raise KeyError(
+            raise UnknownDatabaseError(
                 f"unknown database(s) {', '.join(sorted(set(unknown)))}; this "
                 f"client covers {', '.join(sorted(covered)) or 'a single unnamed database'}"
             )
@@ -864,7 +865,7 @@ class HaikuRAG:
         if not sources:
             return []
         if sources != [self.source]:
-            raise KeyError(
+            raise UnknownDatabaseError(
                 f"unknown database(s) {', '.join(sources) or '(none)'}; this "
                 f"client covers {self.source or 'a single unnamed database'}"
             )
diff --git a/haiku_rag_slim/haiku/rag/client/scope.py b/haiku_rag_slim/haiku/rag/client/scope.py
index 0aa15068..973fd804 100644
--- a/haiku_rag_slim/haiku/rag/client/scope.py
+++ b/haiku_rag_slim/haiku/rag/client/scope.py
@@ -2,7 +2,10 @@ from dataclasses import dataclass
 from pathlib import Path
 
 from haiku.rag.config import AppConfig
-from haiku.rag.store.exceptions import AmbiguousDatabaseError
+from haiku.rag.store.exceptions import (
+    AmbiguousDatabaseError,
+    UnknownDatabaseError,
+)
 from haiku.rag.utils import locate_database
 
 
@@ -95,7 +98,7 @@ class DatabaseScope:
 
         if database_name is not None:
             if database_name not in declared:
-                raise AmbiguousDatabaseError(
+                raise UnknownDatabaseError(
                     f"unknown database {database_name!r}; lancedb.databases names "
                     f"{', '.join(sorted(declared)) or 'nothing'}"
                 )
@@ -129,7 +132,7 @@ class DatabaseScope:
         by_name = {ref.name: ref for ref in self.databases if ref.name is not None}
         missing = [name for name in names if name not in by_name]
         if missing:
-            raise KeyError(
+            raise UnknownDatabaseError(
                 f"unknown database(s) {', '.join(sorted(missing))}; "
                 f"configured: {', '.join(sorted(by_name))}"
             )
diff --git a/haiku_rag_slim/haiku/rag/client/session.py b/haiku_rag_slim/haiku/rag/client/session.py
index ce33033c..3db23cc4 100644
--- a/haiku_rag_slim/haiku/rag/client/session.py
+++ b/haiku_rag_slim/haiku/rag/client/session.py
@@ -12,6 +12,7 @@ from haiku.rag.store.exceptions import (
     MigrationRequiredError,
     ReadOnlyError,
     SourceUnavailableError,
+    UnknownDatabaseError,
 )
 from haiku.rag.store.repositories.chunk import ChunkRepository
 from haiku.rag.store.repositories.document import DocumentRepository
@@ -288,7 +289,7 @@ class FederatedSession:
         """
         unknown = [name for name in names if name not in self._refs]
         if unknown:
-            raise KeyError(
+            raise UnknownDatabaseError(
                 f"unknown database(s) {', '.join(sorted(unknown))}; configured: "
                 f"{', '.join(sorted(self._refs))}"
             )
diff --git a/haiku_rag_slim/haiku/rag/ingester/cli.py b/haiku_rag_slim/haiku/rag/ingester/cli.py
index f8f1eee3..ed359897 100644
--- a/haiku_rag_slim/haiku/rag/ingester/cli.py
+++ b/haiku_rag_slim/haiku/rag/ingester/cli.py
@@ -36,6 +36,7 @@ from haiku.rag.store.exceptions import (  # noqa: E402
     AmbiguousDatabaseError,
     MigrationRequiredError,
     ReadOnlyError,
+    UnknownDatabaseError,
 )
 
 if TYPE_CHECKING:
@@ -71,7 +72,12 @@ def cli() -> None:
     """Entry point that translates store-state errors into a clean exit."""
     try:
         _cli()
-    except (AmbiguousDatabaseError, MigrationRequiredError, ReadOnlyError) as e:
+    except (
+        AmbiguousDatabaseError,
+        MigrationRequiredError,
+        ReadOnlyError,
+        UnknownDatabaseError,
+    ) as e:
         typer.echo(f"Error: {e}", err=True)
         sys.exit(1)
 
diff --git a/haiku_rag_slim/haiku/rag/store/__init__.py b/haiku_rag_slim/haiku/rag/store/__init__.py
index e6126a10..110fa48b 100644
--- a/haiku_rag_slim/haiku/rag/store/__init__.py
+++ b/haiku_rag_slim/haiku/rag/store/__init__.py
@@ -5,6 +5,7 @@ from .exceptions import (
     MigrationRequiredError,
     ReadOnlyError,
     SourceUnavailableError,
+    UnknownDatabaseError,
 )
 from .models import Chunk, Document
 
@@ -17,4 +18,5 @@ __all__ = [
     "AmbiguousDatabaseError",
     "ConfigMismatchError",
     "SourceUnavailableError",
+    "UnknownDatabaseError",
 ]
diff --git a/haiku_rag_slim/haiku/rag/store/exceptions.py b/haiku_rag_slim/haiku/rag/store/exceptions.py
index 81cc4ae2..7b00a639 100644
--- a/haiku_rag_slim/haiku/rag/store/exceptions.py
+++ b/haiku_rag_slim/haiku/rag/store/exceptions.py
@@ -24,6 +24,19 @@ class AmbiguousDatabaseError(Exception):
     """
 
 
+class UnknownDatabaseError(KeyError):
+    """A name that does not belong to the databases in question.
+
+    A `KeyError`, since selecting by name is a lookup, and one type wherever a
+    name is checked: at construction, per query, and when placing evidence.
+    """
+
+    def __str__(self) -> str:
+        # KeyError quotes its argument, which reads as a missing dict key rather
+        # than the sentence this carries.
+        return str(self.args[0]) if self.args else ""
+
+
 class AmbiguousCitationError(Exception):
     """A cited chunk id names a chunk in more than one database.
 
diff --git a/tests/multi_db/test_capabilities.py b/tests/multi_db/test_capabilities.py
index b8359b00..fd530fe9 100644
--- a/tests/multi_db/test_capabilities.py
+++ b/tests/multi_db/test_capabilities.py
@@ -10,6 +10,7 @@ from haiku.rag.client import HaikuRAG
 from haiku.rag.client.scope import DatabaseScope
 from haiku.rag.client.session import FederatedSession
 from haiku.rag.sandbox import AnalysisContext, Sandbox
+from haiku.rag.store.exceptions import UnknownDatabaseError
 from haiku.rag.store.models import SearchResult
 from tests.multi_db.helpers import (
     _config,
@@ -212,7 +213,7 @@ class TestNamingDatabasesBeforeTheModelRuns:
         await _seed(config, "beta", ["beta document about cats"])
 
         async with HaikuRAG(config=config) as rag:
-            with pytest.raises(KeyError, match="typo"):
+            with pytest.raises(UnknownDatabaseError, match="typo"):
                 await rag.ask("what about cats?", sources=["typo"])
 
     @pytest.mark.asyncio
@@ -221,7 +222,7 @@ class TestNamingDatabasesBeforeTheModelRuns:
         await _seed(config, "alpha", ["alpha document about cats"])
 
         async with HaikuRAG(config=config) as rag:
-            with pytest.raises(KeyError, match="typo"):
+            with pytest.raises(UnknownDatabaseError, match="typo"):
                 await rag.analyze("how many?", sources=["typo"])
 
     @pytest.mark.asyncio
@@ -238,7 +239,7 @@ class TestNamingDatabasesBeforeTheModelRuns:
             rag._require_known_sources(None)
             rag._require_known_sources(["alpha"])
             rag._require_known_sources([])
-            with pytest.raises(KeyError, match="typo"):
+            with pytest.raises(UnknownDatabaseError, match="typo"):
                 rag._require_known_sources(["alpha", "typo"])
 
             assert rag._session._sessions == {}
diff --git a/tests/multi_db/test_scope.py b/tests/multi_db/test_scope.py
index aee4bb74..2f5e93d0 100644
--- a/tests/multi_db/test_scope.py
+++ b/tests/multi_db/test_scope.py
@@ -6,7 +6,10 @@ from pydantic import ValidationError
 from haiku.rag.client import HaikuRAG
 from haiku.rag.client.scope import DatabaseScope
 from haiku.rag.config.models import AppConfig, LanceDBConfig
-from haiku.rag.store.exceptions import AmbiguousDatabaseError
+from haiku.rag.store.exceptions import (
+    AmbiguousDatabaseError,
+    UnknownDatabaseError,
+)
 from haiku.rag.utils import locate_database
 from tests.multi_db.helpers import (
     _config,
@@ -147,7 +150,7 @@ class TestSelection:
     async def test_unknown_source_at_construction_is_rejected(self, tmp_path):
         config = _config(tmp_path, ["alpha", "beta"])
 
-        with pytest.raises(KeyError, match="nope"):
+        with pytest.raises(UnknownDatabaseError, match="nope"):
             async with HaikuRAG(config=config, sources=["nope"]):
                 pass
 
@@ -158,7 +161,7 @@ class TestSelection:
         await _seed(config, "beta", ["beta document about cats"])
 
         async with HaikuRAG(config=config) as rag:
-            with pytest.raises(KeyError, match="nope"):
+            with pytest.raises(UnknownDatabaseError, match="nope"):
                 await rag.search("cats", search_type="fts", sources=["nope"])
 
     @pytest.mark.asyncio
@@ -238,6 +241,23 @@ class TestPlacingADatabase:
         async with HaikuRAG(temp_db_path, create=True) as rag:
             assert await rag.reader_for(None) is rag
 
+    def test_one_type_answers_for_a_name_nothing_covers(self, tmp_path):
+        """Selecting by name is a lookup wherever it happens, so it fails the
+        same way at construction, per query, and when placing evidence."""
+        config = _config(tmp_path, ["alpha", "beta"])
+
+        with pytest.raises(UnknownDatabaseError):
+            DatabaseScope.resolve(config, database_name="typo")
+        with pytest.raises(UnknownDatabaseError):
+            DatabaseScope.resolve(config).select(["typo"])
+
+        # A KeyError, so a caller treating selection as a lookup still catches it.
+        assert issubclass(UnknownDatabaseError, KeyError)
+        # ...but the message reads as a sentence rather than a missing key.
+        assert str(UnknownDatabaseError("unknown database 'typo'")) == (
+            "unknown database 'typo'"
+        )
+
     def test_a_path_and_sources_cannot_both_choose(self, tmp_path):
         """`sources` used to be ignored beside a path, so selecting a database
         that is not the one at the path opened the path anyway."""
@@ -257,14 +277,14 @@ class TestPlacingADatabase:
 
         async with HaikuRAG(config=config, sources=["alpha"]) as alpha:
             assert await alpha.reader_for("alpha") is alpha
-            with pytest.raises(KeyError, match="beta"):
+            with pytest.raises(UnknownDatabaseError, match="beta"):
                 await alpha.reader_for("beta")
 
     @pytest.mark.asyncio
     async def test_an_unnamed_database_refuses_any_name(self, temp_db_path):
         """Nothing names it, so no name can be the one it covers."""
         async with HaikuRAG(temp_db_path, create=True) as rag:
-            with pytest.raises(KeyError, match="single unnamed database"):
+            with pytest.raises(UnknownDatabaseError, match="single unnamed database"):
                 await rag.reader_for("anything")
 
     @pytest.mark.asyncio
diff --git a/tests/multi_db/test_search.py b/tests/multi_db/test_search.py
index d56144c8..b18eabf9 100644
--- a/tests/multi_db/test_search.py
+++ b/tests/multi_db/test_search.py
@@ -10,6 +10,7 @@ from haiku.rag.config import get_config
 from haiku.rag.store.exceptions import (
     ConfigMismatchError,
     SourceUnavailableError,
+    UnknownDatabaseError,
 )
 from haiku.rag.store.models import Chunk, DocumentItem
 from tests.multi_db.helpers import (
@@ -54,7 +55,7 @@ class TestFederatedSearch:
         await _seed(config, "alpha", ["alpha document about cats"])
 
         async with HaikuRAG(config=config) as rag:
-            with pytest.raises(KeyError, match="nope"):
+            with pytest.raises(UnknownDatabaseError, match="nope"):
                 await rag.search("cats", search_type="fts", sources=["nope"])
 
     @pytest.mark.asyncio
diff --git a/tests/test_cli.py b/tests/test_cli.py
index fcc847bb..cf8cee30 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -15,6 +15,7 @@ from haiku.rag.config.models import AppConfig, LanceDBConfig, StorageConfig
 from haiku.rag.store.exceptions import (
     AmbiguousDatabaseError,
     MigrationRequiredError,
+    UnknownDatabaseError,
 )
 from tests.conftest import for_path
 
@@ -207,14 +208,14 @@ class TestSelectingADatabaseByName:
         self._install(monkeypatch, alpha="/data/a.lancedb", beta="/data/b.lancedb")
         monkeypatch.setattr("haiku.rag.cli._db_name", "gamma")
 
-        with pytest.raises(AmbiguousDatabaseError, match="alpha, beta"):
+        with pytest.raises(UnknownDatabaseError, match="alpha, beta"):
             resolve_scope(None)
 
     def test_an_unknown_name_does_not_leak_locations(self, monkeypatch):
         self._install(monkeypatch, papers="s3://bucket/prefix/papers.lancedb")
         monkeypatch.setattr("haiku.rag.cli._db_name", "gamma")
 
-        with pytest.raises(AmbiguousDatabaseError) as raised:
+        with pytest.raises(UnknownDatabaseError) as raised:
             resolve_scope(None)
 
         assert "bucket" not in str(raised.value)
@@ -223,7 +224,7 @@ class TestSelectingADatabaseByName:
         self._install(monkeypatch)
         monkeypatch.setattr("haiku.rag.cli._db_name", "papers")
 
-        with pytest.raises(AmbiguousDatabaseError, match="nothing"):
+        with pytest.raises(UnknownDatabaseError, match="nothing"):
             resolve_scope(None)
 
     def test_the_callback_selects_before_a_command_runs(self, tmp_path, monkeypatch):
@@ -240,7 +241,7 @@ class TestSelectingADatabaseByName:
         )
 
         assert result.exit_code != 0
-        assert isinstance(result.exception, AmbiguousDatabaseError)
+        assert isinstance(result.exception, UnknownDatabaseError)
         assert "nope" in str(result.exception)
 
     def test_a_selection_does_not_outlive_its_invocation(self, tmp_path, monkeypatch):
diff --git a/tests/test_database_scope.py b/tests/test_database_scope.py
index b1b83d1b..668061f0 100644
--- a/tests/test_database_scope.py
+++ b/tests/test_database_scope.py
@@ -5,7 +5,10 @@ import pytest
 from haiku.rag.client.scope import DatabaseRef, DatabaseScope
 from haiku.rag.client.session import default_db_path
 from haiku.rag.config.models import AppConfig, LanceDBConfig, StorageConfig
-from haiku.rag.store.exceptions import AmbiguousDatabaseError
+from haiku.rag.store.exceptions import (
+    AmbiguousDatabaseError,
+    UnknownDatabaseError,
+)
 
 
 def _config(**kwargs) -> AppConfig:
@@ -45,7 +48,7 @@ class TestResolution:
     def test_an_unknown_name_is_refused(self):
         config = _config(databases={"alpha": "/data/alpha.lancedb"})
 
-        with pytest.raises(AmbiguousDatabaseError, match="unknown database 'nope'"):
+        with pytest.raises(UnknownDatabaseError, match="unknown database 'nope'"):
             DatabaseScope.resolve(config, database_name="nope")
 
     def test_no_selector_covers_the_configured_set_in_order(self):