TM4C1294NCPDT: TIVA UDP send example

I am trying to send a UDP packet from the TIVA board back to the pc. I am using the UDP echo example and I can do this OK. But I want to be able to send a packet from my main function rather than from the UDP receive function. But when I do this the udp_sendto function crashes. Below is the code. The addr and port vars are what I have saved from the receive function. Anyone know what the issue is? Thanks

void eth_udp_tx()
{
    struct pbuf *p;
    uint8_t buf[16];
    err_t wr_err = ERR_OK;

    p = pbuf_alloc(PBUF_TRANSPORT, 16, PBUF_POOL);
    p->len = 16;
    p->tot_len = 16;
    p->payload = buf;

    if (p)
    {
        wr_err = udp_sendto(new_udp_pcb, p, udp_addr, udp_port);
        pbuf_free(p);
    }
}