What does ? status in htop mean?

Question

I see ? in the S column of htop output. But the manpage only has the following states. What does ? mean? Thanks.

   STATE (S) 
        The state of the process:
           S for sleeping (idle)
           R for running
           D for disk sleep (uninterruptible)
           Z for zombie (waiting for parent to read its exit status)
           T for traced or suspended (e.g by SIGTSTP)
           W for paging