Removing context-param for JCR_URL in web.xml in the rest war causes NPE

I thought the web apps, rest and webdav used the service loader for obtaining the repositories. However, when the JCR_URL parm is removed, the NPE is caused:
Caused by: java.lang.NullPointerException
at org.modeshape.jcr.JcrRepositoryFactory.urlFor(JcrRepositoryFactory.java:421)
at org.modeshape.jcr.JcrRepositoryFactory.getRepositories(JcrRepositoryFactory.java:475)
at org.modeshape.web.jcr.spi.FactoryRepositoryProvider.getRepository(FactoryRepositoryProvider.java:71)
at org.modeshape.web.jcr.spi.FactoryRepositoryProvider.getSession(FactoryRepositoryProvider.java:108)
at org.modeshape.web.jcr.RepositoryFactory.getSession(RepositoryFactory.java:90)
at org.modeshape.web.jcr.rest.AbstractHandler.getSession(AbstractHandler.java:40)
at org.modeshape.web.jcr.rest.RepositoryHandler.getWorkspaces(RepositoryHandler.java:36)
at org.modeshape.web.jcr.rest.JcrResources.getWorkspaces(JcrResources.java:188)