diff --git a/builders/server/core/runtime/validator.py b/builders/server/core/runtime/validator.py
index b59ce5d..6837f87 100644
--- a/builders/server/core/runtime/validator.py
+++ b/builders/server/core/runtime/validator.py
@@ -23,7 +23,17 @@ 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(
+            f"Builder output expected a list of rows, got '{type(data_list).__name__}'"
+        )
+
+    for index, data in enumerate(data_list):
+        if not isinstance(data, dict):
+            raise ValidationError(
+                f"Builder output row {index} expected a dict, "
+                f"got '{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..c85fae8 100644
--- a/builders/server/tests/core/runtime/test_validator.py
+++ b/builders/server/tests/core/runtime/test_validator.py
@@ -82,6 +82,20 @@ def test_validate_rows_empty_list() -> None:
     validate_rows([], {"ticker": SchemaType.STR})
 
 
+@pytest.mark.parametrize("data", [None, {}])
+def test_validate_rows_rejects_non_list_output(data: object) -> None:
+    """Builder output must be a list of row dictionaries."""
+    with pytest.raises(ValidationError, match="expected a list of rows"):
+        validate_rows(data, {"ticker": SchemaType.STR})
+
+
+@pytest.mark.parametrize("row", [None, "bad", 123])
+def test_validate_rows_rejects_non_dict_rows(row: object) -> None:
+    """Each builder output row must be a dictionary."""
+    with pytest.raises(ValidationError, match="row 0 expected a dict"):
+        validate_rows([row], {"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'"):
diff --git a/dev-docs/SPEC-backend.md b/dev-docs/SPEC-backend.md
index f9afdb0..eda4d82 100644
--- a/dev-docs/SPEC-backend.md
+++ b/dev-docs/SPEC-backend.md
@@ -421,6 +421,7 @@ Builders are stateless Python scripts. To each dataset there is a builder script
 
 The `[schema]` section in `config.toml` is used for runtime validation:
 - After a builder returns its output list, the builder server validates each dict in the list against the schema before inserting into the DB.
+- Builder output must be a list of row dictionaries; non-list output fails validation before row-level schema checks.
 - Validation checks that all declared keys are present and that values match the declared types.
 - Validation correctness is the priority over performance.
 - The builder script for dataset `(dataset_name, dataset_version)` is under `builders/scripts/dataset_name/dataset_version/builder.py`. The config is stored under `builders/scripts/dataset_name/dataset_version/config.toml`.