diff --git a/wp-admin/includes/ms.php b/wp-admin/includes/ms.php
index d00c847d68..b9d691a378 100644
--- a/wp-admin/includes/ms.php
+++ b/wp-admin/includes/ms.php
@@ -389,7 +389,7 @@ function get_space_allowed() {
 	if ( ! is_numeric( $space_allowed ) )
 		$space_allowed = get_site_option( 'blog_upload_space' );
 
-	if ( ! is_numeric( $space_allowed ) )
+	if ( empty( $space_allowed ) || ! is_numeric( $space_allowed ) )
 		$space_allowed = 50;
 
 	return $space_allowed;