array check

includes/class-wp-xmlrpc-client.php

@@ -146,7 +146,7 @@ class WP_XMLRPC_Client extends WP_HTTP_IXR_Client implements WP_Client {
         if( empty( $post_thumbnail_id ) )
             return '';
 
-        if( !empty( $this->ext_thumbnail_ids[ $this->site_ID ] ) ) {
+        if( is_array( $this->ext_thumbnail_ids[ $this->site_ID ] ) ) {
             if( array_key_exists( $post_thumbnail_id, $this->ext_thumbnail_ids[ $this->site_ID ] ) )
                 return $this->ext_thumbnail_ids[ $this->site_ID ][ $post_thumbnail_id ];
         }