Index: src/wp-includes/plugin.php
===================================================================
--- src/wp-includes/plugin.php	(revision 28752)
+++ src/wp-includes/plugin.php	(working copy)
@@ -618,7 +618,9 @@
  */
 function plugin_basename( $file ) {
 	global $wp_plugin_paths;
-
+	if ( ! isset( $file ) ) {
+			return;
+	}
 	foreach ( $wp_plugin_paths as $dir => $realdir ) {
 		if ( strpos( $file, $realdir ) === 0 ) {
 			$file = $dir . substr( $file, strlen( $realdir ) );