Seg Fault in ZeroMQ 2.1.4 on zmq_msg_size()

encoder.cpp

bool zmq::encoder_t::message_ready ()
{
    //  Destroy content of the old message.
    zmq_msg_close (&in_progress);

    //  Read new message. If there is none, return false.
    //  Note that new state is set only if write is successful. That way
    //  unsuccessful write will cause retry on the next state machine
    //  invocation.
    if (!source || !source->read (&in_progress)) {
        zmq_msg_init (&in_progress);
        return false;
    }

    //  Get the message size.
    size_t size = zmq_msg_size (&in_progress);       // <- line 68

    // ...
}

GDB backtrace of crash

(gdb) bt
#0  zmq_msg_size (msg_=0x16e53c68) at zmq.cpp:208
#1  0x00002b84d9f5de3b in zmq::encoder_t::message_ready (this=0x16e53c20) at encoder.cpp:68
#2  0x00002b84d9f65ffb in zmq::encoder_base_t<zmq::encoder_t>::get_data (this=0x16e53c00) at encoder.hpp:80
#3  zmq::pgm_sender_t::out_event (this=0x16e53c00) at pgm_sender.cpp:165
#4  0x00002b84d9f62c66 in zmq::object_t::process_command (this=0x15996360, cmd_=...) at object.cpp:63
#5  0x00002b84d9f5fafc in zmq::io_thread_t::in_event (this=<value optimized out>) at io_thread.cpp:83
#6  0x00002b84d9f5e3a2 in zmq::epoll_t::loop (this=0x237bb40) at epoll.cpp:161
#7  0x00002b84d9f73c46 in thread_routine (arg_=0x237bbb0) at thread.cpp:71
#8  0x00002b84da9f39ca in start_thread () from /lib/libpthread.so.0
#9  0x00002b84da75070d in clone () from /lib/libc.so.6

zmq.cpp

size_t zmq_msg_size (zmq_msg_t *msg_)
{
    if (msg_->content == (zmq::msg_content_t*) ZMQ_VSM)
        return msg_->vsm_size;
    if (msg_->content == (zmq::msg_content_t*) ZMQ_DELIMITER)
        return 0;

    return ((zmq::msg_content_t*) msg_->content)->size;     // <- line 208
}