Why does using the wrong format specifier in C crash my program on Windows 7?

My program is as follows;

#include <stdio.h>
#include <string.h>

int main()
{
        char string[] = "Gentlemen start your engines!";
        printf("That string is %s characters long.\r\n", strlen(string));
        return 0;
}

I'm compiling under gcc, and although it doesn't give me any errors the program crashes every time I run it. The code seems to be fine from examples I've seen. It'd be great to know if I'm doing anything wrong.

Thanks.