Index: src/wp-includes/plugin.php
===================================================================
--- src/wp-includes/plugin.php	(revision 27522)
+++ src/wp-includes/plugin.php	(working copy)
@@ -617,9 +617,11 @@
 function plugin_basename( $file ) {
 	global $wp_plugin_paths;
 
-	foreach ( $wp_plugin_paths as $dir => $realdir ) {
-		if ( strpos( $file, $realdir ) === 0 ) {
-			$file = $dir . substr( $file, strlen( $realdir ) );
+	if ( is_array( $wp_plugin_paths ) ) {
+		foreach ( $wp_plugin_paths as $dir => $realdir ) {
+			if ( strpos( $file, $realdir ) === 0 ) {
+				$file = $dir . substr( $file, strlen( $realdir ) );
+			}
 		}
 	}