kill does not allow 0 as a signal

[gcp]

   "If sig is 0, then no signal is sent, but error checking is still performed; this can be used to  check
   for the existence of a process ID or process group ID."

signal.rs seems to explicitly disallow the signal value to be 0, i.e.:

    pub fn from_c_int(signum: libc::c_int) -> Result<Signal> {
        match 0 < signum && signum < NSIG {

[fiveop]

We have not released it yet, but in the next version you can pass None to kill in order to pass no signal. (see #441 and #445).