edit

src/Api.php

@@ -74,12 +74,15 @@ class Api
      */
     public function string($cate)
     {
-        $cate = explode(',', $cate);
-        $result = array();
-        foreach ($cate as $k => $v) {
-            $result[$k] = $this->getName($v);
+        if ($cate) {
+            $cate = explode(',', $cate);
+            $result = array();
+            foreach ($cate as $k => $v) {
+                $result[$k] = $this->getName($v);
+            }
+            return implode(',', $result);
         }
-        return implode(',', $result);
+        return '';
     }