call to XStream constructor with Class Mapper results in Null Pointer

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.