Invalid ARN Prefix, when using Execution ID in GetExecutionHistory

I'm trying to use the context object Execution ID as described here:

https://docs.aws.amazon.com/step-functions/latest/dg/input-output-contextobject.html

So I can call the getExecutionHistory API as a task in my step function so I can keep track of how many events happen and not go over the 25,000 threshold. However when I pass the $$.Execution.Id as a parameter to the API I get an Invalid ARN Prefix error.

What should I be doing differently? Can you not use the API to get a running Step Function's State Machine Event History?