diff --git a/frigate/api/media.py b/frigate/api/media.py
index 476c8349f..9aac3d7e6 100644
--- a/frigate/api/media.py
+++ b/frigate/api/media.py
@@ -909,7 +909,7 @@ def event_thumbnail(
         elif extension == "webp":
             quality_params = [int(cv2.IMWRITE_WEBP_QUALITY), 60]
 
-        _, img = cv2.imencode(f".{img}", thumbnail, quality_params)
+        _, img = cv2.imencode(f".{extension}", thumbnail, quality_params)
         thumbnail_bytes = img.tobytes()
 
     return Response(