using dd for clean MBR code doesn't work on pfSense

Trying to clean the mbr code part on a disk using the pfsense 2.7.0 live disk (pfsense is based on freebsd) under shell command. being **/dev/da0** my drive following the suggested code for clean just the mbr code keeping the partitions the command should be: dd if=/dev/zero of=/dev/da0 bs=446 count=1 however... the result is: dd: /dev/da0: Invalid argument 1+0 records in 0+0 records out 0 bytes transferred in 0.000089 secs (0 bytes/sec) instead... if I use as code just dd if=/dev/zero of=/dev/da0 it just erases everything without errors :( I'm doing this tests in a vm so I can recover the hd many times to test this passage... however this thing is giving me headaches... **EDIT:** It seems that if I use bs=512 or bs=1M it doesn't gives errors. However doing so also the partitions table part would be deleted... **EDIT2:** I tried to use the command dd if=/dev/da0 of=/tmp/mbr_file bs=512 count=1 and it create for me a file with the mbr, I wonder what commands can I use for edit in binary mode the file filling the first 446 bytes with 0 and then use dd if=/tmp/mbr_file of=/dev/da0 bs=512 count=1 to restore it. What could I use? vi?