[Discuss-gnuradio] len_tag_key

Dear all ,
I use a flowgraph containing : File source , OFDM TX ,throttle, File sink (I will replace then the file sink by UHD sink to try with USRP)
I got this error message

FATAL: Missing a required length tag on port 0 at item #0
thread[thread-per-block[1]: <block crc32_bb (14)>]: Missing length tag.

I tried to solve this problem but I failed :

in TX in the field called length tag name ,

I tried  to write same input to this field as in the loopback :

I wrote in this field : len_tag_key

Then I put a variable block with ID :len_tag_key , with value : "packet_len"

Then I put another variable block with ID: packet_len , with value:87

Also I tried to write in this field : packet_len & put only one variable block with ID packet_len & value:87 .. I got the same error !!

What is must be written in this field ?!!!

Thanks.