maven release branch problems

maven.log

~/projects/agnes/[(agnes-1.5)]>mvn release:branch -DbranchName=agnes-1.5.x -DautoVersionSubmodules=true -DreleaseVersion=1.5.1 -DupdateBranchVersions=true                          tfnico@Thomas-Ferris-Nicolaisens-iMac [12:10:23]
[INFO] Scanning for projects...
[INFO]                                                                         
[INFO] ------------------------------------------------------------------------
[INFO] Building agnes 1.5
[INFO] ------------------------------------------------------------------------
[INFO] 
[INFO] --- maven-release-plugin:2.2.1:branch (default-cli) @ agnes ---
[INFO] Verifying that there are no local modifications...
[INFO]   ignoring changes on: pom.xml.next, release.properties, pom.xml.releaseBackup, pom.xml.backup, pom.xml.branch, pom.xml.tag
[INFO] Executing: /bin/sh -c cd /Users/tfnico/projects/agnes && git status
[INFO] Working directory: /Users/tfnico/projects/agnes
[INFO] Transforming 'agnes'...
[INFO] Checking in modified POMs...
[INFO] Executing: /bin/sh -c cd /Users/tfnico/projects/agnes && git add -- pom.xml
[INFO] Working directory: /Users/tfnico/projects/agnes
[INFO] Executing: /bin/sh -c cd /Users/tfnico/projects/agnes && git status
[INFO] Working directory: /Users/tfnico/projects/agnes
[INFO] Executing: /bin/sh -c cd /Users/tfnico/projects/agnes && git commit --verbose -F /var/folders/t4/t4EvRKSaHc0HDjkTqXy5kU+++TI/-Tmp-/maven-scm-180884689.commit pom.xml
[INFO] Working directory: /Users/tfnico/projects/agnes
[INFO] Executing: /bin/sh -c cd /Users/tfnico/projects/agnes && git symbolic-ref HEAD
[INFO] Working directory: /Users/tfnico/projects/agnes
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 1.416s
[INFO] Finished at: Wed Nov 09 12:12:53 CET 2011
[INFO] Final Memory: 4M/81M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal org.apache.maven.plugins:maven-release-plugin:2.2.1:branch (default-cli) on project agnes: An error is occurred in the checkin process: Exception while executing SCM command. Detecting the current branch failed: fatal: ref HEAD is not a symbolic ref -> [Help 1]
[ERROR] 
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR] 
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/MojoExecutionException