Question on nullwhentrue annotation

[riso]

Hi all:
        I`m a new user of the splint. In the manual of splint, I found
an example:
        /*@nullwhentrue@*/ bool isNull ( /*@null@*/ char *x );
        ......
        when I use splint to parse the c file, splint told me:
         Inconsistent function declaration : bool :
        truenull ? 
        why?
Thanks you in advance,
riso