See http://musicbrainz.org/ws/2/discid/tL3Ugj9xojIDXfuUxqfmfkhommM- => response is just blank.

It should either return and empty XML document:
<?xml version="1.0" encoding="UTF-8"?><metadata xmlns="http://musicbrainz.org/ns/mmd-2.0#" xmlns:ext="http://musicbrainz.org/ns/ext-2.0#"></metadata>

or as done by release end-point:
http://musicbrainz.org/ws/2/release/3596119d-c484-490a-a2ea-be86a8f29071
<error><text>Not Found</text><text>For usage, please see: http://musicbrainz.org/development/mmd</text></error>