[Discuss-gnuradio] OFDM receiver error, FATAL: Missing a required length tag on port 0 at item #0

Hi,

I implemented OFDM receiver on USRP E110 using the OFDM receiver in examples in gnuradio's files 3.7

and I added to it a chain to create OFDM transmitter: random source, OFDM transmitter, channel model,  throttle, USRP sink

But when I executed the flow graph. It generated "UOOOOOOOOOOOOOOOOOO" and I got an error :

FATAL: Missing a required length tag on port 0 at item #0 

thread[thread-per-block[21]: <gr_block crc 32_bb (9)>]: Missing length tag. 

and tag debug didn't show anything.

Have you an idea how to fix this error ? 

Thanks in advance