diff --git a/mongoengine/fields.py b/mongoengine/fields.py
index 8692e3a7..d75286f0 100644
--- a/mongoengine/fields.py
+++ b/mongoengine/fields.py
@@ -1108,8 +1108,8 @@ class ReferenceField(BaseField):
         if self.document_type._meta.get('abstract') and \
                 not isinstance(value, self.document_type):
             self.error(
-                '%s is not an instance of abstract reference type %s' % (value,
-                    self.document_type._class_name)
+                '%s is not an instance of abstract reference type %s' % (
+                    value, self.document_type._class_name)
             )
 
     def lookup_member(self, member_name):