diff --git a/llama_stack/apis/vector_io/vector_io.py b/llama_stack/apis/vector_io/vector_io.py
index 4a09a5230..1b16d2832 100644
--- a/llama_stack/apis/vector_io/vector_io.py
+++ b/llama_stack/apis/vector_io/vector_io.py
@@ -96,14 +96,14 @@ class Chunk(BaseModel):
         """Returns the document_id from either metadata or chunk_metadata, with metadata taking precedence."""
         # Check metadata first (takes precedence)
         doc_id = self.metadata.get("document_id")
-        if isinstance(doc_id, str):
+        if doc_id is not None:
+            if not isinstance(doc_id, str):
+                raise TypeError(f"metadata['document_id'] must be a string, got {type(doc_id).__name__}: {doc_id!r}")
             return doc_id
 
-        # Fall back to chunk_metadata if available
+        # Fall back to chunk_metadata if available (Pydantic ensures type safety)
         if self.chunk_metadata is not None:
-            chunk_doc_id = getattr(self.chunk_metadata, "document_id", None)
-            if isinstance(chunk_doc_id, str):
-                return chunk_doc_id
+            return self.chunk_metadata.document_id
 
         return None
 
diff --git a/tests/unit/providers/vector_io/test_vector_io_openai_vector_stores.py b/tests/unit/providers/vector_io/test_vector_io_openai_vector_stores.py
index 12f1fb946..3c1981f49 100644
--- a/tests/unit/providers/vector_io/test_vector_io_openai_vector_stores.py
+++ b/tests/unit/providers/vector_io/test_vector_io_openai_vector_stores.py
@@ -132,6 +132,18 @@ async def test_insert_chunks_with_missing_document_id(vector_io_adapter):
     fake_index.insert_chunks.assert_awaited_once()
 
 
+async def test_document_id_with_invalid_type_raises_error():
+    """Ensure TypeError is raised when document_id is not a string."""
+    from llama_stack.apis.vector_io import Chunk
+
+    # Integer document_id should raise TypeError
+    chunk = Chunk(content="test", metadata={"document_id": 12345})
+    with pytest.raises(TypeError) as exc_info:
+        _ = chunk.document_id
+    assert "metadata['document_id'] must be a string" in str(exc_info.value)
+    assert "got int" in str(exc_info.value)
+
+
 async def test_query_chunks_calls_underlying_index_and_returns(vector_io_adapter):
     expected = QueryChunksResponse(chunks=[Chunk(content="c1")], scores=[0.1])
     fake_index = AsyncMock(query_chunks=AsyncMock(return_value=expected))