Custom Terminology - Validation Rule MD0102

I am interested in utilizing P21's Custom Terminology feature to store metadata about our company specific CT standards.  As is common in the industry, I've created a custom codelist which is a subset of the NCI NY codelist to store just "Y" values, and I've named it NY_Y.  When this is uploaded to P21, the MD0102 rule (Codelist ID value cannot start with a number and cannot include any special characters.) fires.  Why is this the case?  Underscore characters in codelist names has never been an issue in Defines.