diff --git a/alkatorapi/admin.py b/alkatorapi/admin.py
index 0b10bb2..2a2e6de 100644
--- a/alkatorapi/admin.py
+++ b/alkatorapi/admin.py
@@ -20,7 +20,7 @@ class InvoiceAdmin(admin.ModelAdmin):
 
     def response_change(self, request, obj):
         if "_invoice" in request.POST:
-            return HttpResponseRedirect("/api/invoice?invoice_id=" + str(obj.invoice_id))
+            return HttpResponseRedirect(f"/api/invoice?invoice_id={obj.invoice_id}")
         return super(self).response_change(request, obj)