diff --git a/mongoengine/queryset/queryset.py b/mongoengine/queryset/queryset.py
index 65703c32..4c664614 100644
--- a/mongoengine/queryset/queryset.py
+++ b/mongoengine/queryset/queryset.py
@@ -1095,11 +1095,12 @@ class QuerySet(object):
     def _cursor_args(self):
         cursor_args = {
             'snapshot': self._snapshot,
-            'timeout': self._timeout,
-            'slave_okay': self._slave_okay,
+            'timeout': self._timeout
         }
         if self._read_preference is not None:
             cursor_args['read_preference'] = self._read_preference
+        else:
+            cursor_args['slave_okay'] = self._slave_okay
         if self._loaded_fields:
             cursor_args['fields'] = self._loaded_fields.as_dict()
         return cursor_args