Index: sites/all/modules/project_issue/includes/mail.inc
<+>UTF-8
===================================================================
--- sites/all/modules/project_issue/includes/mail.inc	(revision 2fdd2d7ed836f62225f955dc40b4a009d526bb73)
+++ sites/all/modules/project_issue/includes/mail.inc	(revision )
@@ -219,6 +219,11 @@
   // some punctuation) are used.  See example in Appendix A.1.2.
   $from = '"' . mime_header_encode($sender->name) . "\" <$sender->mail>";
 
+  #If notifications are disabled for all users, project_issue_notification_user_settings_load_multiple fails with $recipients null
+  if (!sizeof($recipients)){
+    return;
+  }
+
   // Pre-load the e-mail notification preferences for all users in 1 query.
   $user_preferences = project_issue_notification_user_settings_load_multiple(array_keys($recipients));