diff --git a/builders/server/core/runtime/validator.py b/builders/server/core/runtime/validator.py
index b59ce5d..0386a03 100644
--- a/builders/server/core/runtime/validator.py
+++ b/builders/server/core/runtime/validator.py
@@ -23,7 +23,15 @@ def validate(data: dict, schema: dict[str, SchemaType]) -> None:
             )
 
 
-def validate_rows(data_list: list[dict], schema: dict[str, SchemaType]) -> None:
+def validate_rows(data_list: object, schema: dict[str, SchemaType]) -> None:
     """Validate each dict in a list against the declared schema."""
-    for data in data_list:
+    if not isinstance(data_list, list):
+        raise ValidationError("Builder output must be a list of rows")
+
+    for index, data in enumerate(data_list):
+        if not isinstance(data, dict):
+            raise ValidationError(
+                f"Builder output row {index} must be a dict, got "
+                f"'{type(data).__name__}'"
+            )
         validate(data, schema)
diff --git a/builders/server/tests/core/runtime/test_validator.py b/builders/server/tests/core/runtime/test_validator.py
index d8ee66c..dfea28c 100644
--- a/builders/server/tests/core/runtime/test_validator.py
+++ b/builders/server/tests/core/runtime/test_validator.py
@@ -82,6 +82,19 @@ def test_validate_rows_empty_list() -> None:
     validate_rows([], {"ticker": SchemaType.STR})
 
 
+@pytest.mark.parametrize("data_list", [None, {}])
+def test_validate_rows_rejects_non_list_output(data_list: object) -> None:
+    """Builder outputs must be lists of row dictionaries."""
+    with pytest.raises(ValidationError, match="must be a list of rows"):
+        validate_rows(data_list, {"ticker": SchemaType.STR})
+
+
+def test_validate_rows_rejects_non_dict_item() -> None:
+    """Every row in the builder output must be a dictionary."""
+    with pytest.raises(ValidationError, match="row 1 must be a dict"):
+        validate_rows([{"ticker": "AAPL"}, "MSFT"], {"ticker": SchemaType.STR})
+
+
 def test_validate_rows_invalid_item_raises() -> None:
     """Invalid item in the list raises ValidationError."""
     with pytest.raises(ValidationError, match="Missing key 'price'"):