buildbot failure in on wicket-branch-7.x

[buildbot]

The Buildbot has detected a new failure on builder wicket-branch-7.x while 
building wicket. Full details are available at:
    https://ci.apache.org/builders/wicket-branch-7.x/builds/309

Buildbot URL: https://ci.apache.org/

Buildslave for this Build: bb_slave1_ubuntu

Build Reason: The SingleBranchScheduler scheduler named 
'on-wicket-branch-7.x-commit' triggered this build
Build Source Stamp: [branch wicket-7.x] 9aff085e4387222ac48ea5984d7e5141f5c0faa8
Blamelist: Artur Michałowski <amichalowsk...@gmail.com>

BUILD FAILED: failed compile

Sincerely,
 -The Buildbot