Why MatLab gives me incorrect result for prime number?

The function isprime() gives correct result for "small" numbers:

isprime(2^31-1) TRUE

BUT it gives incorrect result for larger numbers that are proved to be prime:

isprime(2^61-1) FALSE (but should be true)

WHY??

OK, I guess it is because I am trying to use a bigger number than Matlab can handle. But then, why Matlab does not show any error? and how can I check if a number this big is prime using MatLab?