Hello @MikeB, based on the issue you are facing, and this other question in the community, it seems like you are specifically hitting a limitation related to multi-value parameters and functionality with NULLs.
I was led to this conclusion because if you wanted to check a multi-value parameter for the select all option in a calculated field, you check if it is returning a NULL value:
in(NULL, ${Parameter})
I would imagine, the error you are experiencing would be eliminated if you switched from a multi-value to a single value option. If that is not a possibility, switching a true NULL value for a string NULL may be the best work-around for you at the moment.
I would also recommend filing a case with AWS Support where we can dive into the details so that we can help you further. Here are the steps to open a support case. If your company has someone who manages your AWS account, you might not have direct access to AWS
Support and will need to raise an internal ticket to your IT team or whomever manages your AWS account. They should be able to open an AWS Support case on your behalf. Hope this helps!