"The JIRA server was contacted but has returned an error response" when using "Where is my field?" helper on Create Issue screen

Steps to reproduce

1. Click Create Issue.
2. Choose "Configure Fields", select "Where is my field?".
3. Select a field.

Result

You'll immediately get a "The JIRA server was contacted but has returned an error response. We are unsure of the result of this operation."

The Javascript console reports this:

[Error] Failed to load resource: https://<instance>/rest/whereismycf/1.0/fields/creator the server responded with a status of 400 (Bad Request)

The content of that request:

Either 'issueKey', or 'projectKey' or 'projectId' parameter must be provided in the request

This appears to only happen in the Create Issue screen, it works fine when viewing an already-existing issue.