usernames passed to api.php are not normalized. lowercase first letter guarantees empty result set
compare:
http://en.wikipedia.org/w/api.php?action=query&list=allusers&aulimit=1&auprop=groups|editcount&auprefix=yurik
http://en.wikipedia.org/w/api.php?action=query&list=allusers&aulimit=1&auprop=groups|editcount&auprefix=Yurik
Version: 1.11.x
Severity: trivial
URL: http://en.wikipedia.org/w/api.php?action=query&list=allusers&aulimit=1&auprop=groups|editcount&auprefix=yurik