diff --git a/Bugzilla/BugMail.pm b/Bugzilla/BugMail.pm
index 18795d735..2075a71e1 100644
--- a/Bugzilla/BugMail.pm
+++ b/Bugzilla/BugMail.pm
@@ -495,10 +495,10 @@ sub _generate_bugmail {
   }
 
   my $email = Bugzilla::MIME->new($msg_header);
-  if (scalar(@parts) == 1) {
-    $email->content_type_set($parts[0]->content_type);
-  }
-  else {
+
+  # If there's only one part, we don't need to set the overall content type
+  # because Email::MIME will automatically take it from that part (bug 1657496)
+  if (scalar(@parts) > 1) {
     $email->content_type_set('multipart/alternative');
 
     # Some mail clients need same encoding for each part, even empty ones.