Message too long

This should be a relatively simple problem, but it is beyond me.  I'm trying 
to write 1243 bytes to the wire (which should fit in an ethernet frame), but 
am getting "Message too long".  

This function (below) is getting called, and it is printing:

1243
ERROR (libnet_context.cxx,249,0x8104fa0): Write error: libnet_write_link(): 
only -1 bytes written (Message too long)

Here is the function:

// Send a Base packet

bool LibnetContext::send(BasePacket& base_packet)
{
    libnet_ptag_t ip_tag = LIBNET_PTAG_INITIALIZER,
                  ethernet_tag = LIBNET_PTAG_INITIALIZER,
                  udp_tag = LIBNET_PTAG_INITIALIZER;

    const string contents=base_packet.serialize_contents();
    const int length=contents.length();

    cout << (LIBNET_IPV4_H + LIBNET_UDP_H + length) << endl;

    //Build the udp header
    udp_tag = libnet_build_udp(
             base_packet.src_port.value(),
             base_packet.dest_port.value(),
             LIBNET_UDP_H,
             0,
             (u_int8_t*)contents.data(),                //payload
             length,                    //payload size
             l,                         //libnet handle
             udp_tag);                  //libnet id
    if (udp_tag == -1)
    {
        ERROR << "Can't build UDP header: " << libnet_geterror(l) << "\n";
        return false;
    }

    //Build the ipv4 header
    ip_tag = libnet_build_ipv4(
             LIBNET_IPV4_H + LIBNET_UDP_H + length,  //length
             0,                 //TOS
             0,                 //IP ID
             0,                 //IP Frag       
            64,                //TTL
             IPPROTO_UDP,       //Protocol
             0,                 //checksum
             base_packet.src_ip.get_inaddr(),   //source IP
             base_packet.dest_ip.get_inaddr(),  //destination IP
             NULL,              //payload
             0,                 //payload size
             l,                 //libnet handle
             ip_tag);           //libnet id
    if (ip_tag == -1)
    {
        ERROR << "Can't build IP header: " << libnet_geterror(l) << "\n";
        return false;
    }

    //Build the ethernet header

    ethernet_tag = libnet_build_ethernet(
             base_packet.dest_mac.get_array(),
             base_packet.src_mac.get_array(),
             ETHERTYPE_IP,
             NULL,
             0,
             l,
             ethernet_tag);

    if (ethernet_tag == -1)
    {
        ERROR << "Can't build ethernet header: " << libnet_geterror(l) << 
"\n";
        return false;
   }

    const int c = libnet_write(l);
    if (c==-1)
    {
        ERROR << "Write error: " << libnet_geterror(l) << "\n";
        return false;
    }

    return true;
}

ERROR is a macro that prints the header on the error message; line 249 is the 
line that says "Write error...". 

For anyone who can offer any help, I'd be greatly appreciative.  :)

 - Karen