diff --git a/src/ytdl_sub/script/script.py b/src/ytdl_sub/script/script.py
index 8523cbd8..771f51ee 100644
--- a/src/ytdl_sub/script/script.py
+++ b/src/ytdl_sub/script/script.py
@@ -9,6 +9,7 @@ from ytdl_sub.script.types.syntax_tree import SyntaxTree
 from ytdl_sub.script.types.variable import Variable
 from ytdl_sub.script.utils.exceptions import UNREACHABLE
 from ytdl_sub.script.utils.exceptions import CycleDetected
+from ytdl_sub.script.utils.name_validation import validate_variable_name
 
 # pylint: disable=missing-raises-doc
 
@@ -97,7 +98,7 @@ class Script:
             self._function_name(name) for name in overrides.keys() if self._is_function(name)
         }
         variable_names: Set[str] = {
-            name for name in overrides.keys() if not self._is_function(name)
+            validate_variable_name(name) for name in overrides.keys() if not self._is_function(name)
         }
 
         self._functions: Dict[str, SyntaxTree] = {
diff --git a/src/ytdl_sub/script/utils/name_validation.py b/src/ytdl_sub/script/utils/name_validation.py
index 462bed58..d1dd1488 100644
--- a/src/ytdl_sub/script/utils/name_validation.py
+++ b/src/ytdl_sub/script/utils/name_validation.py
@@ -16,7 +16,7 @@ def is_valid_name(name: str) -> bool:
     return re.match(_NAME_REGEX_VALIDATOR, name) is not None
 
 
-def validate_variable_name(variable_name: str) -> None:
+def validate_variable_name(variable_name: str) -> str:
     """
     Raises
     ------
@@ -25,10 +25,18 @@ def validate_variable_name(variable_name: str) -> None:
     """
     if not is_valid_name(variable_name):
         raise InvalidVariableName(
-            f"Variable name '{variable_name}' is invalid. "
-            f"Names must be lower_snake_cased and begin with a letter."
+            f"Variable name '{variable_name}' is invalid:"
+            " Names must be lower_snake_cased and begin with a letter."
         )
 
+    if Functions.is_built_in(variable_name):
+        raise InvalidVariableName(
+            f"Variable name '{variable_name}' is invalid:"
+            " The name is used by a built-in function and cannot be overwritten."
+        )
+
+    return variable_name
+
 
 def validate_custom_function_name(custom_function_name: str) -> None:
     """
diff --git a/tests/unit/script/types/test_variable.py b/tests/unit/script/types/test_variable.py
index 071ba881..59eb6f66 100644
--- a/tests/unit/script/types/test_variable.py
+++ b/tests/unit/script/types/test_variable.py
@@ -52,12 +52,72 @@ class TestVariable:
         ):
             Script({"a": "a", "b": "{c}"}).resolve()
 
-    def test_invalid_variable_name_inline(self):
+    @pytest.mark.parametrize(
+        "name",
+        [
+            "vali_LOL_INVALID",
+            "name!!",
+            "na(",
+            "na[",
+        ],
+    )
+    def test_invalid_variable_name_inline(self, name: str):
         with pytest.raises(
             InvalidVariableName,
             match=re.escape(
-                "Variable name 'vali_LOL_INVALID' is invalid. "
-                "Names must be lower_snake_cased and begin with a letter."
+                f"Variable name '{name}' is invalid:"
+                " Names must be lower_snake_cased and begin with a letter."
             ),
         ):
-            Script({"a": "{vali_LOL_INVALID}"}).resolve()
+            Script({"a": f"{{{name}}}"}).resolve()
+
+    @pytest.mark.parametrize(
+        "name",
+        ["float", "bool", "mul"],
+    )
+    def test_invalid_variable_name_inline_is_built_in(self, name: str):
+        with pytest.raises(
+            InvalidVariableName,
+            match=re.escape(
+                f"Variable name '{name}' is invalid:"
+                " The name is used by a built-in function and cannot be overwritten."
+            ),
+        ):
+            Script({"a": f"{{{name}}}"}).resolve()
+
+    @pytest.mark.parametrize(
+        "name",
+        [
+            "vali_LOL_INVALID",
+            "name!!",
+            "na(",
+            "na[",
+            "$2232",
+            "1245",
+            "CAN_CATCH_MORE",
+            "{brackets_in_definition}",
+        ],
+    )
+    def test_invalid_variable_name_definition(self, name: str):
+        with pytest.raises(
+            InvalidVariableName,
+            match=re.escape(
+                f"Variable name '{name}' is invalid:"
+                " Names must be lower_snake_cased and begin with a letter."
+            ),
+        ):
+            Script({f"{name}": "value"}).resolve()
+
+    @pytest.mark.parametrize(
+        "name",
+        ["float", "bool", "mul"],
+    )
+    def test_invalid_variable_name_definition_is_built_in(self, name: str):
+        with pytest.raises(
+            InvalidVariableName,
+            match=re.escape(
+                f"Variable name '{name}' is invalid:"
+                " The name is used by a built-in function and cannot be overwritten."
+            ),
+        ):
+            Script({f"{name}": "value"}).resolve()