diff --git a/nummi/main/models.py b/nummi/main/models.py
index b1662d6..23a22ba 100644
--- a/nummi/main/models.py
+++ b/nummi/main/models.py
@@ -83,6 +83,7 @@ class Invoice(models.Model):
 
 class InvoiceForm(ModelForm):
     template_name = "main/form.html"
+    prefix = "invoice"
 
     class Meta:
         model = Invoice