diff --git a/modoboa/admin/forms/domain.py b/modoboa/admin/forms/domain.py
index 00ac7c962..93653368a 100644
--- a/modoboa/admin/forms/domain.py
+++ b/modoboa/admin/forms/domain.py
@@ -97,6 +97,7 @@ class DomainFormGeneral(forms.ModelForm, DynamicForm):
                 % unicode(label)
             )
         condition = (
+            self.cleaned_data["quota"] != 0 and
             self.cleaned_data["default_mailbox_quota"] >
             self.cleaned_data["quota"])
         if condition:
diff --git a/modoboa/admin/serializers.py b/modoboa/admin/serializers.py
index 3376c73a6..5c6aee1b9 100644
--- a/modoboa/admin/serializers.py
+++ b/modoboa/admin/serializers.py
@@ -29,8 +29,8 @@ class DomainSerializer(serializers.ModelSerializer):
 
     def validate(self, data):
         """Check quota values."""
-        quota = data["quota"]
-        default_mailbox_quota = data["default_mailbox_quota"]
+        quota = data.get("quota", 0)
+        default_mailbox_quota = data.get("default_mailbox_quota", 0)
         if quota != 0 and default_mailbox_quota > quota:
             raise serializers.ValidationError({
                 "default_mailbox_quota":