A question in sdsIncrLen

In the function sdsIncrLen, I think the statement "assert(sh->free >= 0) " is redundant, because "assert(sh->free >= incr)" makes sure this situation would not happen.

void sdsIncrLen(sds s, int incr) {
struct sdshdr _sh = (void_)(s - sizeof *sh);
assert(sh->free >= incr);
sh->len += incr;
sh->free -= incr;
assert(sh->free >= 0);
s[sh->len] = '\0';
}