-make_alt (LitAlt l,_,e) = C.Alit (make_lit l) (make_exp e)
-make_alt (DEFAULT,[],e) = C.Adefault (make_exp e)
+make_alt (LitAlt l,_,e) = C.Alit (make_lit l) (make_exp e)
+make_alt (DEFAULT,[],e) = C.Adefault (make_exp e)
+-- This should never happen, as the DEFAULT alternative binds no variables,
+-- but we might as well check for it:
+make_alt a@(DEFAULT,_ ,_) = pprPanic ("MkExternalCore: make_alt: DEFAULT "
+ ++ "alternative had a non-empty var list") (ppr a)
+