do_coerce modifies x, so stashing the original before seems to fix the problem. Maybe I missed something else. Lightly-tested patch: --- a/numeric.c +++ b/numeric.c @@ -3426,10 +3426,11 @@ static int bit_coerce(VALUE *x, VALUE *y) { if (!FIXNUM_P(*y) && !RB_TYPE_P(*y, T_BIGNUM)) { + VALUE orig = *x; do_coerce(x, y, TRUE); if (!FIXNUM_P(*x) && !RB_TYPE_P(*x, T_BIGNUM) && !FIXNUM_P(*y) && !RB_TYPE_P(*y, T_BIGNUM)) { - coerce_failed(*x, *y); + coerce_failed(orig, *y); } } return TRUE;