As texted inside the Issue:
It depends on the used extension for the call.
/api/v1/accounts/verify_credentials.json
returns a JSON error, /api/v1/accounts/verify_credentials
doesn't.
But it's like Twitter expects it as well.
Try
api.twitter.com/1.1/account/ve… (empty page)
api.twitter.com/1.1/account/ve… (JSON Error)