credstash-grant-reader could not be planned with empty(default) context_keys and context_values

Checking out the code, this line looks weird to me. https://github.com/fpco/terraform-aws-foundation/blob/d15768b3499638bba76c878e83659faa71741876/modules/credstash-grant-reader/main.tf#L54

Seems like I must pass one and only one key/value pair for each role. Wondering if this code was not as it supposed to be.