Index: wp-includes/plugin.php
===================================================================
--- wp-includes/plugin.php	(revision 5811)
+++ wp-includes/plugin.php	(working copy)
@@ -57,7 +57,7 @@
 				$string = call_user_func_array($the_['function'], array_slice($args, 1, (int) $the_['accepted_args']));
 			}
 
-	} while ( next($wp_filter[$tag]) );
+	} while ( next($wp_filter[$tag]) !== false );
 
 	return $string;
 }