diff --git a/apps/qm/serializers.py b/apps/qm/serializers.py
index 98f13d2a..a1dbeff4 100644
--- a/apps/qm/serializers.py
+++ b/apps/qm/serializers.py
@@ -373,6 +373,8 @@ class FtestProcessSerializer(CustomModelSerializer):
                 FtestDefect.objects.create(ftest=instance, **item2)
             if not is_ok:
                 instance.defect_main = defect_main
+            else:
+                instance.defect_main = None
             instance.is_ok = is_ok
             instance.save()
         return instance
@@ -415,6 +417,8 @@ class FtestProcessSerializer(CustomModelSerializer):
                     ins.save()
             if not is_ok:
                 instance.defect_main = defect_main
+            else:
+                instance.defect_main = None
             instance.is_ok = is_ok
             instance.save()
         return instance
\ No newline at end of file