diff --git a/youtube_dl/postprocessor/embedthumbnail.py b/youtube_dl/postprocessor/embedthumbnail.py
index 699eeca0c..3523f4992 100644
--- a/youtube_dl/postprocessor/embedthumbnail.py
+++ b/youtube_dl/postprocessor/embedthumbnail.py
@@ -8,7 +8,7 @@ try:
     import imghdr
     from mutagen.mp4 import MP4, MP4Cover, MP4MetadataError
 except ImportError:
-    raise Exception('[embedthumbnail] Mutagen isn\'t found, install from PyPI.')
+    raise Warning('[embedthumbnail] Mutagen isn\'t found for M4A thumbnail embedding, install from PyPI.')
 
 from .ffmpeg import FFmpegPostProcessor