Index: wp-includes/plugin.php
===================================================================
--- wp-includes/plugin.php	(revision 20950)
+++ wp-includes/plugin.php	(working copy)
@@ -399,7 +399,7 @@
 
 	do {
 		foreach ( (array) current($wp_filter[$tag]) as $the_ )
-			if ( !is_null($the_['function']) )
+			if ( is_callable($the_['function']) )
 				call_user_func_array($the_['function'], array_slice($args, 0, (int) $the_['accepted_args']));
 
 	} while ( next($wp_filter[$tag]) !== false );