No Container Instances were found in your cluster

When trying to run the Fargate custom executor driver, I'm getting this issue:

INFO[2020-05-18T20:23:35Z] Starting fargate                              PID=1570 version="0.1.0 (bc3d342)"
INFO[2020-05-18T20:23:35Z] Executing the command                         PID=1570 command=config_exec
Using Custom executor with driver fargate 0.1.0 (bc3d342)...
INFO[2020-05-18T20:23:35Z] Starting fargate                              PID=1578 version="0.1.0 (bc3d342)"
INFO[2020-05-18T20:23:35Z] Executing the command                         PID=1578 command=prepare_exec
INFO[2020-05-18T20:23:35Z] Starting new Fargate task                     PID=1578 command=prepare_exec
ERRO[2020-05-18T20:23:36Z] Application execution failed                  PID=1578 error="starting new Fargate task: running new task on Fargate: error starting AWS Fargate Task: InvalidParameterException: No Container Instances were found in your cluster."
WARNING: Cleanup script failed: exit status 2
ERROR: Preparation failed: exit status 2

My cluster doesn't have any EC2 instances and I don't want to add any. I also don't have a capacity provider strategy since they seem primarily focused on supporting EC2. It feels like the custom executor could safely hardcode the LaunchType to FARGATE in the task creation call since this is specifically called the "Fargate custom executor". Of course the other possibility is that I've simply done something wrong, so if anyone else has seen this error, I'd love to know what the workaround is.