Ubus_connect FD 0 (zero)

Hi! I have the case that sometimes ubus_connect() is successful, but when I later get the filedescriptor via ctx->ubus->sock.fd (I integrate UBUS with another event loop) the filedescriptor is 0 (zero). I am wondering if this is an error or not, as I believe that FD zero is usually stdin... on the other hand the code in ubus looks like 0 is valid... Normally I get a nonzero FD, when I first connect to UBUS, the zero FD only comes some times after I ubus_free/ubus_connect... Can anyone explain? Any ideas?