The slot coercing that is implemented in set-slot-value-using-class.lisp relies on slot setting always going through (setf closer-mop:slot-value-using-class). LispWorks by default optimizes slots access so it doesn't.
To force writing through (setf closer-mop:slot-value-using-class) , classes need to specify :optimize-slot-access nil in the class options. See: http://www.lispworks.com/documentation/lw71/LW/html/lw-389.htm#pgfId-1039223.
As it is, the test test-slot-value-coersion fails on LispWorks.