diff --git a/youtube_dl/utils.py b/youtube_dl/utils.py
index f333e4712..7bcc85e2b 100644
--- a/youtube_dl/utils.py
+++ b/youtube_dl/utils.py
@@ -1935,6 +1935,9 @@ def error_to_compat_str(err):
 
 
 def mimetype2ext(mt):
+    if mt is None:
+        return None
+
     ext = {
         'audio/mp4': 'm4a',
     }.get(mt)