Haproxy external-check error code 255

I'm quite new with Haproxy and I have a weird behavior with external check..

here is my config file :

global
    log /dev/log    local0
    log /dev/log    local1 notice
    chroot /var/lib/haproxy
    stats socket /run/haproxy/admin.sock mode 660 level admin
    stats timeout 30s
    user haproxy
    group haproxy
    daemon

    # Default SSL material locations
    ca-base /etc/ssl/certs
    crt-base /etc/ssl/private

    # Default ciphers to use on SSL-enabled listening sockets.
    # For more information, see ciphers(1SSL). This list is from:
    #  https://hynek.me/articles/hardening-your-web-servers-ssl-ciphers/
    ssl-default-bind-ciphers ECDH+AESGCM:DH+AESGCM:ECDH+AES256:DH+AES256:ECDH+AES128:DH+AES:ECDH+3DES:DH+3DES:RSA+AESGCM:RSA+AES:RSA+3DES:!aNULL:!MD5:!DSS
    ssl-default-bind-options no-sslv3
    external-check

defaults
    log    global
    mode    http
    option    httplog
    option    dontlognull
        timeout connect 5000
        timeout client  50000
        timeout server  50000
    errorfile 400 /etc/haproxy/errors/400.http
    errorfile 403 /etc/haproxy/errors/403.http
    errorfile 408 /etc/haproxy/errors/408.http
    errorfile 500 /etc/haproxy/errors/500.http
    errorfile 502 /etc/haproxy/errors/502.http
    errorfile 503 /etc/haproxy/errors/503.http
    errorfile 504 /etc/haproxy/errors/504.http

backend web-backend
    balance roundrobin
    option external-check
    external-check path "/usr/bin:/bin:/tmp"
    external-check command /bin/true
    server web1 127.0.0.1:80 check
    server web2 127.0.0.1:81 check

frontend http
    bind *:82
    mode http

    default_backend web-backend

And yet I get a 503 and in my logs I got this :

...: Server web-backend/web1 is DOWN, reason: External check error, code: 255, check duration: 5ms. 1 active and 0 backup servers left. 0 sessions active, 0 requeued, 0 remaining in queue.
...: Server web-backend/web1 is DOWN, reason: External check error, code: 255, check duration: 5ms. 1 active and 0 backup servers left. 0 sessions active, 0 requeued, 0 remaining in queue.
...: Server web-backend/web2 is DOWN, reason: External check error, code: 255, check duration: 1ms. 0 active and 0 backup servers left. 0 sessions active, 0 requeued, 0 remaining in queue.
...: Server web-backend/web2 is DOWN, reason: External check error, code: 255, check duration: 1ms. 0 active and 0 backup servers left. 0 sessions active, 0 requeued, 0 remaining in queue.
...: backend web-backend has no server available!
...: backend web-backend has no server available!

How could the check /bin/true fail and return 255 ?

I'm running the latest stable version (1.6.5)

Thanks !

EDIT I'm on Debian8 and I used this : https://haproxy.debian.net/ to install the latest version (1.6.5)

Turns out I was using a jail.. Which was empty ! So there was no binary to execute in the jail (chroot /var/lib/haproxy).