diff --git a/prestodb/client.py b/prestodb/client.py
index 1bf82fe..2d6203d 100644
--- a/prestodb/client.py
+++ b/prestodb/client.py
@@ -547,7 +547,7 @@ def execute(self):
         while (
             not self._finished and not self._cancelled
         ):
-            self._result._rows += self.fetch()
+            self._result._rows.extend(self.fetch())
         return self._result
 
     def fetch(self):