Anyone have any insight on this REST API call?

The user guide references a "PUT" to "/assetmgmt/assets/customfields" but when I make the call, I get:

"Message": "The requested resource does not support http method 'PUT'."

as a response. I have several other calls using PUT that are working. This function uses a Content-Type of "x-www-form-urlencoded", which is the only difference from other working calls, which use "application/json". I'm getting this error against both VSA 9.4 and 9.5 platforms.

So far, this is the only call of about 30 that I've tested that's failing.