By definition, a prime number can be a positive integer greater than 1.
Negative numbers can't be primes by definition.
@ikataitsev there is an interesting note about negative prime numbers - http://primes.utm.edu/notes/faq/negative_primes.html
So current code makes sense actually.
Prime numbers can't be negative but prime elements can. There are complex numbers ;)
Closing this, please refer to Bug #7395 in redmine