Index: masquerade.module
===================================================================
RCS file: /cvs/drupal-contrib/contributions/modules/masquerade/masquerade.module,v
retrieving revision 1.16.2.39
diff -u -r1.16.2.39 masquerade.module
--- masquerade.module	23 Jun 2010 13:33:34 -0000	1.16.2.39
+++ masquerade.module	10 Sep 2010 22:10:25 -0000
@@ -40,8 +40,8 @@
   if ($uid) {
     $_SESSION['masquerading'] = $uid;
   }
-  else {
-    $_SESSION['masquerading'] = null;
+  elseif(isset($_SESSION['masquerading'])) {
+    $_SESSION['masquerading'] = NULL;
   }
 }