--- a/mercurial/mail.py	Thu Oct 24 16:56:36 2019 +0200
+++ b/mercurial/mail.py	Thu Oct 24 14:31:24 2019 +0200
@@ -458,7 +458,7 @@
             try:
                 uparts.append(part.decode(charset))
                 continue
-            except UnicodeDecodeError:
+            except (UnicodeDecodeError, LookupError):
                 pass
         # On Python 3, decode_header() may return either bytes or unicode
         # depending on whether the header has =?<charset>? or not