diff --git a/mongoengine/queryset.py b/mongoengine/queryset.py
index 308bdf7f..90547e22 100644
--- a/mongoengine/queryset.py
+++ b/mongoengine/queryset.py
@@ -667,8 +667,8 @@ class QuerySet(object):
                 parts = []
 
                 cleaned_fields = []
-                append_field = True
                 for field in fields:
+                    append_field = True
                     if isinstance(field, str):
                         parts.append(field)
                         append_field = False