Unspecified transport in ACK?

[g.bongiorni@xxxxxxxx (Giorgio Bongiorni)]

Hi Benny and all,
I wish to make a call towards an IP phone.
In "remote_uri" and "target" params of "pjsip_dlg_create_uac" I 
specified the transport (UDP)

    pj_str_t remote_uri = 
pj_str("<sip:201 at 10.13.0.104:5060;transport=UDP>");

and after creating invite session I set the transport for the dialog 
within INVITE session

    pjsip_tpselector tp_sel;    
    tp_sel.type = PJSIP_TPSELECTOR_TRANSPORT;
    tp_sel.u.transport = g_transport;
    pjsip_dlg_set_transport(g_inv->dlg, &tp_sel);

Everything seems to work well, but when ACK is sent I get this message

    Target '10.13.0.104:5060' type=Unspecified resolved to 
'10.13.0.104:5060' type=UDP (UDP transport)

Am I missing anything? Any help is appreciated.
Thank you,

Giorgio Bongiorni