ClassCastException in LazySortedMap when used with equals/transformers

When LazySortedMap is used by equals, the result is
java.lang.ClassCastException: org.apache.commons.collections4.map.LazySortedMap cannot be cast to java.lang.String

This seems to be odd, as the use of the different transformations should not result in internal casting to String. See the attached unit test to reproduce.