Index: wp-admin/includes/ms.php
===================================================================
--- wp-admin/includes/ms.php	(revision 19592)
+++ wp-admin/includes/ms.php	(working copy)
@@ -385,9 +385,10 @@
  */
 function get_space_allowed() {
 	$space_allowed = get_option( 'blog_upload_space' );
-	if ( $space_allowed === false )
+
+	if ( $space_allowed === false || $space_allowed === '' )
 		$space_allowed = get_site_option( 'blog_upload_space' );
-	if ( empty( $space_allowed ) || !is_numeric( $space_allowed ) )
+	if ( $space_allowed === false || $space_allowed === '' || !is_numeric( $space_allowed ) )
 		$space_allowed = 50;
 
 	return $space_allowed;