Stripe webhook testing - 500 error

I have a working production Stripe webhook trigger. I want to be able to test it as well so I imported it to Sandbox. In Sandbox mode however I am getting '500 - Internal' error.

I suspect it is due to the live and not the test key being in the Stripe webhook in Buildship. But then: How do I test Stripe webhook triggers without opening up a new workflow?

Any experience? Thank you!