Create MX_PERSON API Call - MX_MANAGER Failing

Experts,
I'm trying to get MX_PERSON records created via an API call. I have it working, that's the good news. The bad news is that when I try to add a manager to the entry I'm creating, I get a 400 Bad Request with, "Unexpected value for parameter ER_MX_MANAGER", see below screenshot. I'm sending the MSKEY of the entry I want to be this entry's manager.

What am I doing wrong?