diff --git a/backend/open_webui/routers/ollama.py b/backend/open_webui/routers/ollama.py
index 64373c616..4fca10e1f 100644
--- a/backend/open_webui/routers/ollama.py
+++ b/backend/open_webui/routers/ollama.py
@@ -944,7 +944,7 @@ class ChatMessage(BaseModel):
 class GenerateChatCompletionForm(BaseModel):
     model: str
     messages: list[ChatMessage]
-    format: Optional[dict] = None
+    format: Optional[Union[dict, str]] = None
     options: Optional[dict] = None
     template: Optional[str] = None
     stream: Optional[bool] = True