This issue was reported by Péter Salvi via e-mail (thanks Péter!).

SUB-RESOLVE-SELECTION and SUB-DIMENSION both specialize on fixnum, which is not a class that is guaranteed to exist by the standard and is not present in eg ECL or CLISP.

Péter's suggestion: read-time conditionals, eg #+(or ecl clisp) integer #-(or ecl clisp) fixnum. Probably I would just use integer.

However, the issue is moot since I will soon remove SUB as it is superseded by CL-SLICE:SLICE.