Using ngrep & output to dump file

If possible, could someone please tell me what is wrong with the following: ngrep -I ngrep.pcap -i “ocsp” port 80 and dst host -O pcap.pcap_dump.

Any help would be greatly appreciated as I’m being asked to carry out the above, then present the md5sum hash of the pcap.pcap_dump file…all well and good, but I keep getting told that the md5sum hash result is wrong!