diff --git a/library/think/db/Query.php b/library/think/db/Query.php
index a88fc13d..d42b83c5 100644
--- a/library/think/db/Query.php
+++ b/library/think/db/Query.php
@@ -908,9 +908,7 @@ class Query
                 $where = $field;
             } elseif ($field) {
                 // 字符串查询
-                $where[] = ['exp', $field];
-            } else {
-                $where = '';
+                $where[$field] = ['null', ''];
             }
         } elseif (is_array($op)) {
             $where[$field] = $param;