diff --git a/src/utils/filter.py b/src/utils/filter.py
index 8df3590c..f7510797 100644
--- a/src/utils/filter.py
+++ b/src/utils/filter.py
@@ -5,6 +5,7 @@ from typing import Any, TypeVar
 from typing import cast as typing_cast
 
 from sqlalchemy import ColumnElement, Select, and_, case, cast, literal, not_, or_
+from sqlalchemy.dialects.postgresql import JSONB
 from sqlalchemy.types import Numeric
 
 from ..exceptions import FilterError
@@ -345,6 +346,14 @@ def _build_field_condition(
             # For JSONB fields (metadata, configuration), check if it contains nested comparison operators
             if column_name in JSONB_COLUMNS:
                 return _build_nested_metadata_conditions(column, value)  # pyright: ignore
+            elif not isinstance(column.type, JSONB):
+                # A dict against a scalar column compiles fine but fails in the
+                # driver at execute time ("cannot adapt type 'dict'") as a 500.
+                # Reject unknown operator dicts here as a 422 instead.
+                keys = sorted(typing_cast("dict[str, Any]", value))
+                raise FilterError(
+                    f"Invalid filter for column '{key}': unsupported operator(s) {keys}. Expected one of {sorted(COMPARISON_OPERATORS)} or a scalar value."
+                )
             else:
                 return column == value
     else:
diff --git a/tests/utils/test_filter.py b/tests/utils/test_filter.py
new file mode 100644
index 00000000..49f84e23
--- /dev/null
+++ b/tests/utils/test_filter.py
@@ -0,0 +1,31 @@
+"""Unit tests for filter condition building."""
+
+import pytest
+from sqlalchemy import select
+
+from src.exceptions import FilterError
+from src.models import Document
+from src.utils.filter import apply_filter
+
+
+def test_unknown_operator_dict_on_scalar_column_raises():
+    """An unrecognized operator dict must 422, not reach the driver as a 500.
+
+    Regression: {"session_id": {"operator": "null"}} compiled to
+    `session_name = %(param)s` with a dict bind, which psycopg rejected with
+    "cannot adapt type 'dict'" -> unhandled 500.
+    """
+    with pytest.raises(FilterError):
+        apply_filter(select(Document), Document, {"session_id": {"operator": "null"}})
+
+
+def test_known_operator_dict_on_scalar_column_still_works():
+    stmt = apply_filter(
+        select(Document), Document, {"session_id": {"in": ["s1", "s2"]}}
+    )
+    assert "session_name IN" in str(stmt).replace("documents.", "")
+
+
+def test_dict_on_jsonb_column_still_works():
+    stmt = apply_filter(select(Document), Document, {"metadata": {"kind": "note"}})
+    assert "internal_metadata" in str(stmt)