normally, when you add '--' in the list of options, it means to ignore any element beginning with '-' after it and not to parse them as option
this is usefull for example
espeak -- -56
to output as minus fifty six as speak
but when you run something like that on windows, espeak return immediatly and do nothing. the exit status code is 0.
Note: if you use french (-v fr) the negative sign is ignored. One have to use explicitly 'moins' instead of '-' to get in pronounced !