InvalidSerializedDataException when using suggested namespace for Gallery Manager

Steps to reproduce
1) Start archetype with essentials
2) Go to Tools/Gallery Manager
3) Fill in name myimageset and leave namespace to the suggested namespace. Here essentialstest.
4) Create new image set
5) Edit the image set and add image variant bigimage
6) Save
Rebuild & Restart

Expected: CMS is restarted without error
Actual: Error
Error initializing content for jar:file:/media/WORK/svn/essentials/test/essentials-test/target/tomcat7x/webapps/cms/WEB-INF/lib/essentials-test-bootstrap-configuration-1.0-SNAPSHOT.jar!/configuration/frontend/cms.xml in '/hippo:configuration/hippo:frontend' : javax.jcr.InvalidSerializedDataException: illegal node name: essentialstest:bigimage