Index: src/wp-admin/includes/class-wp-upgrader.php
===================================================================
--- src/wp-admin/includes/class-wp-upgrader.php	(revision 26006)
+++ src/wp-admin/includes/class-wp-upgrader.php	(working copy)
@@ -1332,9 +1332,11 @@
 		if ( !isset( $current->response ) || $current->response == 'latest' )
 			return new WP_Error('up_to_date', $this->strings['up_to_date']);
 
-		$res = $this->fs_connect( array(ABSPATH, WP_CONTENT_DIR) );
-		if ( is_wp_error($res) )
+		$res = $this->fs_connect( array( ABSPATH, WP_CONTENT_DIR ) );
+		if ( is_wp_error( $res ) )
 			return $res;
+		elseif ( ! $res )
+			return new WP_Error( 'fs_unavailable', $this->strings['fs_unavailable'] );
 
 		$wp_dir = trailingslashit($wp_filesystem->abspath());