the following 18-bit hashcash is assigned a score of 3, despite being 18 bits:

1:18:250522073955:nullptr#twoblade.com::TQBba1FQFrcjfmpm/JFosQ:AAt5Ag

the official hashcash program verifies it fine:

# hashcash -c -d -r nullptr#twoblade.com 1:18:250522073955:nullptr#twoblade.com::TQBba1FQFrcjfmpm/JFosQ:AAt5Ag -b 18
matched stamp: 1:18:250522073955:nullptr#twoblade.com::TQBba1FQFrcjfmpm/JFosQ:AAt5Ag
check: ok

note that this hashcash will be invalid by the time anyone tests this issue, because the timestamp will be too out of date.