username and password returns an empty response

In some cases (not all) the API will return a successful response with no API key, has anyone else experienced this?

the response is this: 
`<?xml version="1.0" encoding="UTF-8"?>
    <rsp stat="ok" version="1.0">
</rsp>`

where a normal response would be 
`<?xml version="1.0" encoding="UTF-8"?>
    <rsp stat="ok" version="1.0">
        <api_key>8c8c5ed092488fd0354b0d4421a</api_key>
    </rsp>`
