Custom Field renders PDF invalid

[Ric16Fr]

Hey there,
great API, I truly love it!
There is just one issue, when trying to add a order ID as custom field to my invoice.
I used this syntax:
invoice_data["custom_fields"] = [{'name': "Bestellnummer", 'value': order.get_order_id()}] and when printing this out -> print(data['custom_fields'][0]['value']) I also reccieve the correct value or name. But when including this in my request json, the PDF becomes invalid. When commenting this out, everything is fine.

Thanks for helping me out here!
Kind regards
Ric