diff --git a/library/think/db/Query.php b/library/think/db/Query.php
index 0396faca..53aa71b5 100644
--- a/library/think/db/Query.php
+++ b/library/think/db/Query.php
@@ -793,7 +793,7 @@ class Query
         }
 
         if (isset($this->options['field'])) {
-            $field = array_merge($this->options['field'], $field);
+            $field = array_merge((array) $this->options['field'], $field);
         }
         $this->options['field'] = array_unique($field);
         return $this;