gulp-prompt prevents use of sftp

[sbsrouteur]

When using gulp-prompt in a pipe to query a privatekey pasword before sending to gulp-ftp, sftp hangs (happens with all sftp clients I tried gulp-ftp, gulp-sftp-up5, gulp-ssh). (see ).

Use sample is :
`
gulp.task('deploy-test', function () {

sftpOptions = {
host: 'host',
port: 22,
remotePath: '/var/www/Test/',
key: {
location: "private.key",
passphrase: '12345'
},
buffer:false,
username:"webuser",

};
res = gulp.src(['./.php', './dist/**/','./ws/**/*'],{base: './'})
.pipe(prompt.prompt({
type: 'password',
name: 'pass',
message: 'Please enter your password'
}, function(res){
sftpOptions.key.passphrase=res.pass;
}))
.pipe(sftp(sftpOptions));
return res;
});`

When doing so, the ftp will sort of start then hang. Removing the call to prompt and all works properly again.
How is one supposed to call prompt for such a use case?