500 Error: having trouble using IPv6 on ECS with Fargate

I've sent in this bug report to the aws terraform provider, but I'll report it here, too.

When I try to make an ECS service on an IPv6 only subnet, the AWS API sends me back a 500 server error. I can consistently reproduce this error. The details to reproduce it are in the bug report, all you need are the terraform binary and an AWS API key to configure the provider. I'm completely stuck and I don't know what to do.