diff --git a/library/think/db/Query.php b/library/think/db/Query.php
index 1534bf72..f0f0ec39 100644
--- a/library/think/db/Query.php
+++ b/library/think/db/Query.php
@@ -451,7 +451,9 @@ class Query
             if (isset($this->options['field'])) {
                 unset($this->options['field']);
             }
-            if ($key && '*' != $field) {
+            if (is_null($field)) {
+                $field = '*';
+            } elseif ($key && '*' != $field) {
                 $field = $key . ',' . $field;
             }
             $pdo = $this->field($field)->getPdo();