Trouble with Jira Transitions via API

[Joe Osborne]

I am attempting to close Jira issues via REST API, however, I am getting a 500 error when attempting to POST to the transitions API endpoint.

I am submitting the following as JSON:

closeIssue = {
"transition":{
"id": "6"
}
}

Each submission results in the following error:

{"errorMessages":["Internal server error"],"errors":{}}

When I attempt to GET the transitions endpoint, I get the following response:

{"expand":"transitions","transitions":[]}

I think I should be seeing possible transitions with the GET action, but I don't see anything.