diff --git a/rest_api/controller/request.py b/rest_api/controller/request.py
index 9e32d5ffb..d4804842a 100644
--- a/rest_api/controller/request.py
+++ b/rest_api/controller/request.py
@@ -10,7 +10,7 @@ MAX_RECURSION_DEPTH = sys.getrecursionlimit() - 1
 
 class Question(BaseModel):
     questions: List[str]
-    filters: Optional[Dict[str, str]] = None
+    filters: Optional[Dict[str, Optional[str]]] = None
     top_k_reader: int = DEFAULT_TOP_K_READER
     top_k_retriever: int = DEFAULT_TOP_K_RETRIEVER