Zephyr Scale REST API question

Anyone using the REST API with Zephyr Scale trying to update test cases?

I have a set of test cases that I want to add a label to but they are spread all throughout the test case repo so doing it via the UI is not practical.

Looked at using the REST API but when I try to update a test case I get back a 400 error code with a message of "updateTestCase: must not be null".

I know I'm successfully authenticating because I can run GET requests and get back valid responses so there must be something I'm doing wrong on the PUT request to update a test case.

Any help would be appreciated.