diff --git a/apps/wpm/serializers.py b/apps/wpm/serializers.py
index 19e4c697..0873e902 100644
--- a/apps/wpm/serializers.py
+++ b/apps/wpm/serializers.py
@@ -202,7 +202,8 @@ class WMaterialSerializer(CustomModelSerializer):
     
     def to_representation(self, instance):
         ret = super().to_representation(instance)
-        ret['count_cando'] = str(Decimal(ret['count']) - Decimal(ret['count_working']))
+        if 'count' in ret:
+            ret['count_cando'] = str(Decimal(ret['count']) - Decimal(ret['count_working']))
         return ret
 
 class MlogbDefectSerializer(CustomModelSerializer):