Error while using example grc files of ofdm in 3.7 version

hi community,

i tried to use the examples in ofdm. first i tried to simulate by
both ofdm_tx and ofdm_rx in a single flowgraph and i ran it. i got the
following error.

FATAL: Missing a required length tag on port 0 at item #0
thread[thread-per-block[34]: <block ofdm_frame_equalizer_vcvc (22)>]:
Missing length tag.

am i wrong in using these flowgraphs? please correct me… and how to
overcome it. the flowgraph i used is attached.