Move issues to sprint REST API call does not seem to be working

Even when using an open sprint,

POST https:{instanceurl}/rest/agile/1.0/sprint/{sprintId}/issue/{issueId}

call always returns:

<status-code>404</status-code>

And the issue is not added to the sprint.

Tested with JIRA Agile v6.7.9-D20150721T124923