diff --git a/rest_framework/serializers.py b/rest_framework/serializers.py
index ae2e8fcc5..83ee0c8df 100644
--- a/rest_framework/serializers.py
+++ b/rest_framework/serializers.py
@@ -979,15 +979,13 @@ class ModelSerializer(Serializer):
                 m2m_fields.append((attr, value))
             else:
                 setattr(instance, attr, value)
-            else:
-                m2m_fields.append((attr, value))
+
+        instance.save()
 
         for attr, value in m2m_fields:
             field = getattr(instance, attr)
             field.set(value)
 
-        instance.save()
-
         return instance
 
     # Determine the fields to apply...