g_mime_object_set_header (GMIME_OBJECT (reply), "In-Reply-To", in_reply_to);
 
     orig_references = notmuch_message_get_header (message, "references");
-    if (!orig_references)
-       /* Treat errors like missing References headers. */
-       orig_references = "";
-    references = talloc_asprintf (ctx, "%s%s%s",
-                                 *orig_references ? orig_references : "",
-                                 *orig_references ? " " : "",
-                                 in_reply_to);
+    if (orig_references && *orig_references)
+       references = talloc_asprintf (ctx, "%s %s", orig_references,
+                                     in_reply_to);
+    else
+       references = talloc_strdup (ctx, in_reply_to);
+
     g_mime_object_set_header (GMIME_OBJECT (reply), "References", references);
 
     from_addr = add_recipients_from_message (reply, config,