My git + gerrit + husdson + gerrit trigger are apparently configured ok. When a new patch is push to gerrit, hudson is able to compile the patch. The problem happens when I push a commit that is a merge. Here is the log:

Triggered by Gerrit: http://git.touchtec.com.br/gerrit/44
Checkout:workspace / /opt/hudson/jobs/gerrit-sandbox2/workspace - hudson.remoting.LocalChannel@122f55d
Using strategy: Gerrit Hudson Trigger
Last Built Revision: Revision bba35564d8b833615e8e1dfe30d6c09b4ab2b967 (null)
Checkout:workspace / /opt/hudson/jobs/gerrit-sandbox2/workspace - hudson.remoting.LocalChannel@122f55d
GitAPI created
Fetching changes from the remote Git repository
Fetching upstream changes from ssh://hudson@git.touchtec.com.br:29418/sandbox
[workspace] $ git fetch -t ssh://hudson@git.touchtec.com.br:29418/sandbox refs/changes/44/44/1
From ssh://hudson@git.touchtec.com.br:29418/sandbox

• branch refs/changes/44/44/1 -> FETCH_HEAD
  [workspace] $ git ls-tree HEAD
  [workspace] $ git rev-parse FETCH_HEAD
  Commencing build of Revision f731d588331f39933a5ea3739f315a1d30ae621a (null)
  GitAPI created
  Checking out Revision f731d588331f39933a5ea3739f315a1d30ae621a (null)
  [workspace] $ git checkout -f f731d588331f39933a5ea3739f315a1d30ae621a
  [workspace] $ git tag -a -f -m "Hudson Build #32" hudson-gerrit-sandbox2-32
  [workspace] $ git rev-parse FETCH_HEAD
  [workspace] $ git log -1 --pretty=format:%P f731d588331f39933a5ea3739f315a1d30ae621a
  FATAL: Invalid id: bba35564d8b833615e8e1dfe30d6c09b4ab2b967 5f6c3a8772c77e384c2787a998934074c5ffba8e
  java.lang.IllegalArgumentException: Invalid id: bba35564d8b833615e8e1dfe30d6c09b4ab2b967 5f6c3a8772c77e384c2787a998934074c5ffba8e
  at org.spearce.jgit.lib.ObjectId.fromString(ObjectId.java:230)
  at com.sonyericsson.hudson.plugins.gerrit.trigger.hudsontrigger.GerritTriggerBuildChooser.getFirstParent(GerritTriggerBuildChooser.java:108)
  at com.sonyericsson.hudson.plugins.gerrit.trigger.hudsontrigger.GerritTriggerBuildChooser.prevBuildForChangelog(GerritTriggerBuildChooser.java:79)
  at hudson.plugins.git.GitSCM$4.invoke(GitSCM.java:755)
  at hudson.plugins.git.GitSCM$4.invoke(GitSCM.java:707)
  at hudson.FilePath.act(FilePath.java:753)
  at hudson.FilePath.act(FilePath.java:735)
  at hudson.plugins.git.GitSCM.checkout(GitSCM.java:707)
  at hudson.model.AbstractProject.checkout(AbstractProject.java:1038)
  at hudson.model.AbstractBuild$AbstractRunner.checkout(AbstractBuild.java:479)
  at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:411)
  at hudson.model.Run.run(Run.java:1248)
  at hudson.maven.MavenModuleSetBuild.run(MavenModuleSetBuild.java:318)
  at hudson.model.ResourceController.execute(ResourceController.java:88)
  at hudson.model.Executor.run(Executor.java:129)