diff --git a/semantic_router/encoders/bm25.py b/semantic_router/encoders/bm25.py
index 0fc55173c651a5661e4e00b5057f343799247953..451273cdcc78899861c7b64baea4eb4e1cc6b33b 100644
--- a/semantic_router/encoders/bm25.py
+++ b/semantic_router/encoders/bm25.py
@@ -6,7 +6,7 @@ from semantic_router.utils.logger import logger
 
 class BM25Encoder(BaseEncoder):
     model: Optional[Any] = None
-    idx_mapping: dict[int, int] | None = None
+    idx_mapping: Optional[dict[int, int]] = None
     type: str = "sparse"
 
     def __init__(