mpz_invert() limit?

I have been writing a program to generate pairs of RSA keys using small prime numbers. I have been using mpz_invert() to get a value for d for the private key. On small prime numbers it calculates the correct value, but on larger primes it calculates incorrect values.

I'm assuming there must be an upper limit to the values the mpz_invert() function can handle? If so, what is this limit to avoid erroneous values?

Thank You :)
-Sam

Hmm, I've started using even bigger numbers and now it's working again.
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