Index: privatemsg.module
===================================================================
--- privatemsg.module	(revision 346)
+++ privatemsg.module	(working copy)
@@ -277,6 +277,11 @@
 
     if (is_null($account)) {
       global $user;
+      // If the user isnt logged in, let them login instead of showing an 404 page. 
+      if($user->uid == 0) {
+        drupal_goto('user/login', drupal_get_destination());
+        return FALSE;
+      }
       $account = drupal_clone($user);
     }