diff --git a/src/ytdl_sub/script/parser.py b/src/ytdl_sub/script/parser.py
index a6109f4d..3b6644f4 100644
--- a/src/ytdl_sub/script/parser.py
+++ b/src/ytdl_sub/script/parser.py
@@ -498,6 +498,8 @@ class _Parser:
                     raise MAP_KEY_WITH_NO_VALUE
                 if isinstance(key, NonHashable):
                     raise MAP_KEY_NOT_HASHABLE
+                if isinstance(key, BuiltInFunction) and issubclass(key.output_type(), NonHashable):
+                    raise MAP_KEY_NOT_HASHABLE
                 if len(value_args) > 1:
                     raise MAP_KEY_MULTIPLE_VALUES
 
diff --git a/src/ytdl_sub/script/types/function.py b/src/ytdl_sub/script/types/function.py
index 7d1cea85..a3b39a9c 100644
--- a/src/ytdl_sub/script/types/function.py
+++ b/src/ytdl_sub/script/types/function.py
@@ -291,3 +291,6 @@ class BuiltInFunction(Function, BuiltInFunctionType):
             raise FunctionRuntimeException(
                 f"Runtime error occurred when executing the function %{self.name}: {str(exc)}"
             ) from exc
+
+    def __hash__(self):
+        return hash((self.name, *self.args))
diff --git a/src/ytdl_sub/utils/script.py b/src/ytdl_sub/utils/script.py
index 9a9b48af..317adced 100644
--- a/src/ytdl_sub/utils/script.py
+++ b/src/ytdl_sub/utils/script.py
@@ -66,7 +66,9 @@ class ScriptUtils:
             ast = parse(text=value).ast
             if len(ast) == 1:
                 return ast[0]
-            return BuiltInFunction(name="concat", args=ast)
+            return BuiltInFunction(
+                name="concat", args=[BuiltInFunction(name="string", args=[arg]) for arg in ast]
+            )
         if isinstance(value, int):
             return Integer(value)
         if isinstance(value, float):
diff --git a/tests/unit/script/types/test_map.py b/tests/unit/script/types/test_map.py
index 3a6608cf..576eba35 100644
--- a/tests/unit/script/types/test_map.py
+++ b/tests/unit/script/types/test_map.py
@@ -198,3 +198,16 @@ class TestMap:
                     "key_variable": "{['non-hashable']}",
                 }
             ).resolve()
+
+    def test_map_key_is_function(self):
+        assert Script(
+            {
+                "dict": "{{ %concat('hi', %string(' world')) : 'value' }}",
+                "key_variable": "hashable",
+            }
+        ).resolve() == ScriptOutput(
+            {
+                "key_variable": String("hashable"),
+                "dict": Map(value={String(value="hi world"): String(value="value")}),
+            }
+        )
diff --git a/tests/unit/validators/test_string_formatter_validator.py b/tests/unit/validators/test_string_formatter_validator.py
index 7a0f9f1d..21ecc509 100644
--- a/tests/unit/validators/test_string_formatter_validator.py
+++ b/tests/unit/validators/test_string_formatter_validator.py
@@ -97,6 +97,7 @@ class TestUnstructuredDictFormatterValidator(object):
         key5_bool = True
         key6_map = {"{variable}_key": "value", "static_key": "{variable}_value"}
         key7_list = ["list_1", "list_{variable_2}"]
+        key8_many_vars = "string {variable1} with multiple {variable2}"
         validator = dict_validator_class(
             name="validator",
             value={
@@ -107,17 +108,19 @@ class TestUnstructuredDictFormatterValidator(object):
                 "key5": key5_bool,
                 "key6": key6_map,
                 "key7": key7_list,
+                "key8": key8_many_vars,
             },
         )
 
-        assert len(validator.dict) == 7
+        assert len(validator.dict) == 8
         assert all(isinstance(val, expected_formatter_class) for val in validator.dict.values())
         assert validator.dict_with_format_strings == {
-            "key1": "string with {variable}",
+            "key1": "{ %concat( %string( '''string with ''' ), %string( variable ) ) }",
             "key2": "no variables",
-            "key3": "{%int(3)}",
-            "key4": "{%float(4.132)}",
-            "key5": "{%int(True)}",
-            "key6": '{%from_json(\'\'\'{"static_key": "{variable}_value", "{variable}_key": "value"}\'\'\')}',
-            "key7": "{%from_json('''[\"list_1\", \"list_{variable_2}\"]''')}",
+            "key3": "{ %int(3) }",
+            "key4": "{ %float(4.132) }",
+            "key5": "{ %int(True) }",
+            "key6": "{ { %concat( %string( variable ), %string( '''_key''' ) ): '''value''', '''static_key''': %concat( %string( variable ), %string( '''_value''' ) ) } }",
+            "key7": "{ [ '''list_1''', %concat( %string( '''list_''' ), %string( variable_2 ) ) ] }",
+            "key8": "{ %concat( %string( '''string ''' ), %string( variable1 ), %string( ''' with multiple ''' ), %string( variable2 ) ) }",
         }