Mod operator with negative number and big number is off by one

[batessr]

I am trying to use Truffle Ruby with a project that uses the Jose gem. It gets an error when loading. I have isolated the problem with the mod operator with a negative number and a large number:

Given the following expression:

a = -390081 % ((2**448)-(2**224)-1)

In MRI Ruby and JRuby 10:
a will be:

726838724295606890549323807888004534353641360687318060281490199180612328166730772686396383698676545930088884461843637361053498017975358

In Truffle Ruby:
a will be one less:

726838724295606890549323807888004534353641360687318060281490199180612328166730772686396383698676545930088884461843637361053498017975357

Test code:

a = -390081 % ((2**448)-(2**224)-1)
puts "a #{a}"
puts "MRI Ruby value match #{a==726838724295606890549323807888004534353641360687318060281490199180612328166730772686396383698676545930088884461843637361053498017975358}"