isalpha() giving an assertion

I have a C code in which I am using standard library function isalpha() in ctype.h, This is on Visual Studio 2010-Windows. In below code, if char c is '£', the isalpha call returns an assertion as shown in the snapshot below:

char c='£';

if(isalpha(c))
{
    printf ("character %c is alphabetic\n",c);

}
else
{
    printf ("character %c is NOT alphabetic\n",c);
}

I can see that this might be because 8 bit ASCII does not have this character.

So how do I handle such Non-ASCII characters outside of ASCII table?

What I want to do is if any non-alphabetic character is found(even if it includes such character not in 8-bit ASCII table) i want to be able to neglect it.