Index: wp-includes/functions.php
===================================================================
--- wp-includes/functions.php	(revision 39975)
+++ wp-includes/functions.php	(working copy)
@@ -2054,7 +2054,7 @@
 	if ( $unique_filename_callback && is_callable( $unique_filename_callback ) ) {
 		$filename = call_user_func( $unique_filename_callback, $dir, $name, $ext );
 	} else {
-		$number = '';
+		$number = null;
 
 		// Change '.ext' to lower case.
 		if ( $ext && strtolower($ext) != $ext ) {