`dirname': no implicit conversion of nil into String (TypeError)

[sibelius]

I've tried to run this like this:

gem install ledger_web
createdb ledger
ledger_web

that threw this error:

Traceback (most recent call last):
6: from ~/.rvm/gems/ruby-2.7.2/bin/ruby_executable_hooks:22:in `<main>'
	5: from ~/.rvm/gems/ruby-2.7.2/bin/ruby_executable_hooks:22:in `eval'
4: from ~/.rvm/gems/ruby-2.7.2/bin/ledger_web:23:in `<main>'
	3: from ~/.rvm/gems/ruby-2.7.2/bin/ledger_web:23:in `load'
2: from ~/.rvm/gems/ruby-2.7.2/gems/ledger_web-1.5.2/bin/ledger_web:42:in `<top (required)>'
	1: from ~/.rvm/gems/ruby-2.7.2/gems/ledger_web-1.5.2/lib/ledger_web/watcher.rb:10:in `run!'
~/.rvm/gems/ruby-2.7.2/gems/ledger_web-1.5.2/lib/ledger_web/watcher.rb:10:in `dirname': no implicit conversion of nil into String (TypeError)