diff --git a/kfet/models.py b/kfet/models.py
index 0e3af79b..ff69d0fa 100644
--- a/kfet/models.py
+++ b/kfet/models.py
@@ -186,6 +186,9 @@ class Checkout(models.Model):
     class Meta:
         ordering = ['-valid_to']
 
+    class IsProtected(Exception):
+        pass
+
     def __str__(self):
         return self.name
 
diff --git a/kfet/views.py b/kfet/views.py
index 6f478d7d..529e6053 100644
--- a/kfet/views.py
+++ b/kfet/views.py
@@ -397,7 +397,9 @@ def kpsul_perform_operations(request):
     try:
         checkout_pk = request.POST.get('checkout', 0)
         checkout    = Checkout.objects.get(pk=checkout_pk)
-    except (Checkout.DoesNotExist, ValueError):
+        if checkout.is_protected:
+            raise Checkout.IsProtected
+    except (Checkout.DoesNotExist, ValueError, Checkout.IsProtected):
         data['errors'].append('Caisse invalide')
     if 'errors' in data:
         return JsonResponse(data, status=400)