diff --git a/alkatorapi/admin.py b/alkatorapi/admin.py
index bda0bf8..5eb9a71 100644
--- a/alkatorapi/admin.py
+++ b/alkatorapi/admin.py
@@ -19,7 +19,7 @@ class InvoiceAdmin(admin.ModelAdmin):
 
     def response_change(self, request, obj):
         if "_invoice" in request.POST:
-            return HttpResponseRedirect("/api/invoice?invoice_id=" + obj.invoice_id)
+            return HttpResponseRedirect("/api/invoice?invoice_id=" + str(obj.invoice_id))
         return super(self).response_change(request, obj)