Invalid REST API request setting custom fields raises TypeError

[jeremystretch]

NetBox version

v3.3.0

Python version

3.8

Steps to Reproduce

Send a malformed REST API request attempting to set custom fields for an object. For example:

curl -X PATCH \
-H "Authorization: Token $TOKEN" \
-H "Content-Type: application/json" \
-H "Accept: application/json; indent=4" \
http://netbox:8000/api/dcim/sites/1/ \
--data '{"custom_fields": "xxx"}'

Note that custom_fields maps to a string rather than a dictionary.

Expected Behavior

NetBox should return a 400 Bad Request response indicating the reason for the failure.

Observed Behavior

A TypeError exception is raised.