diff --git a/youtube_dl/postprocessor/embedthumbnail.py b/youtube_dl/postprocessor/embedthumbnail.py
index b6c60e127..9d8934b7f 100644
--- a/youtube_dl/postprocessor/embedthumbnail.py
+++ b/youtube_dl/postprocessor/embedthumbnail.py
@@ -115,7 +115,7 @@ class EmbedThumbnailPP(FFmpegPostProcessor):
             stdout, stderr = process_communicate_or_kill(p)
 
             if p.returncode != 0:
-                msg = stderr.decode('utf-8', 'replace').strip()
+                msg = (stdout + stderr).decode('utf-8', 'replace').strip()
                 raise EmbedThumbnailPPError(msg)
 
             if not self._already_have_thumbnail: