diff --git a/comfy_api/latest/_input_impl/video_types.py b/comfy_api/latest/_input_impl/video_types.py
index a3d48c87f..999894edb 100644
--- a/comfy_api/latest/_input_impl/video_types.py
+++ b/comfy_api/latest/_input_impl/video_types.py
@@ -408,6 +408,10 @@ class VideoFromComponents(VideoInput):
         extra_kwargs = {}
         if isinstance(format, VideoContainer) and format != VideoContainer.AUTO:
             extra_kwargs["format"] = format.value
+        elif isinstance(path, io.BytesIO):
+            # BytesIO has no file extension, so av.open can't infer the format.
+            # Default to mp4 since that's the only supported format anyway.
+            extra_kwargs["format"] = "mp4"
         with av.open(path, mode='w', options={'movflags': 'use_metadata_tags'}, **extra_kwargs) as output:
             # Add metadata before writing any streams
             if metadata is not None: