Sandbox error when trying to update/patch username via Management API

While running a query to PATCH a user’s username, I keep getting a fairly cryptic error about ‘Value’.

url = ""
headers = {
 'Content-Type': 'application/json',
  'Accept': 'application/json',
 'Authorization': f'Bearer {management_api_token}'
data = { "username": "<new_username>" }
response = requests.patch(url, headers=headers, data=data)

{"statusCode":400,"error":"Bad Request","message":"Sandbox Error: Cannot read properties of undefined (reading 'Value')"}

This endpoint works inside of the testing API page here:

For context, I’ve also tried this with the Auth0 python SDK (==4.7.0), with the same error.