Correlate_access_code_tag_bb gives Assertion `px != 0' failed

Hi All,

I am trying to sync to a access code for a bit stream to determine the
start of a packet and pack bits to a byte from that point. I have
challenges getting correlate_access_code_tag_bb working. Attached is a
test grc and every time I try to run it, it gives out
“/target/include/boost/smart_ptr/intrusive_ptr.hpp:162: T*
boost::intrusive_ptr::operator->() const [with T = pmt::pmt_base]:
Assertion `px != 0’ failed” at the point it matches the access code. Any
idea why.

Also is there a better way to start packing bits to a byte at the packet
start boundary