Using an API from < 1.1.2 where a Class Mapper is passed into the XStream constructor now results in a null pointer. To wit: Caused by: java.lang.NullPointerException at com.thoughtworks.xstream.XStream.alias(XStream.java:531) at com.thoughtworks.xstream.XStream.setupAliases(XStream.java:239) at com.thoughtworks.xstream.XStream.<init>(XStream.java:194) at com.thoughtworks.xstream.XStream.<init>(XStream.java:181) The constructor calls setupAliases, which calls alias(String, Class) for a predetermined set of aliases. However, the method alias uses the field 'classAliasingMapper', which is ONLY set in the buildMapper method, which is NOT called when the constructor is given a class mapper.