diff --git a/longs/forms.py b/longs/forms.py
index 233c69cb..d3493c4b 100755
--- a/longs/forms.py
+++ b/longs/forms.py
@@ -269,7 +269,7 @@ LongSeriesImageFormSet = inlineformset_factory(
 class ExamForm(ExamFormMixin, ModelForm):
     class Meta(ExamFormMixin.Meta):
         model = Exam
-        fields = ExamAuthorFormMixin.Meta.fields.append("double_mark")
+        fields = ExamAuthorFormMixin.Meta.fields + ["double_mark"]
 
 class ExamAuthorForm(ExamAuthorFormMixin):
     class Meta(ExamAuthorFormMixin.Meta):