diff --git a/backend/open_webui/retrieval/utils.py b/backend/open_webui/retrieval/utils.py
index 890d0e8929..923ace4bb5 100644
--- a/backend/open_webui/retrieval/utils.py
+++ b/backend/open_webui/retrieval/utils.py
@@ -926,6 +926,12 @@ def get_embedding_function(
     concurrent_requests=0,
 ) -> Awaitable:
     if embedding_engine == '':
+        if embedding_function is None:
+            raise ValueError(
+                'No embedding model is loaded. Set RAG_EMBEDDING_MODEL to a valid '
+                'SentenceTransformer model name, or configure an external '
+                'RAG_EMBEDDING_ENGINE (ollama, openai, azure_openai).'
+            )
         # Sentence transformers: CPU-bound sync operation
         async def async_embedding_function(query, prefix=None, user=None):
             return await asyncio.to_thread(