Just assumed it wasn't a developer issue but rather me doing something wrong. Moving discussion to developer list.
Some improvement!
Parse argument 3:"-m/sw/pkg/pike/src/Pike-v7.7.21-snapshot/build/sunos-5.10-sun4u/master.pike"...
[...]
Init master... There's no master to handle the error. Dumping it raw: Error.BadArgument("Subtract on different types.\n")
Ok, seems the change had the intended effect. Now try updating interpret.c.