Missing LOGSTASH_HOME when running "rake docs:generate-docs"

When I run rake docs:generate-docs from a clean Logstash tree (latest master, commit 00a99c19) I get this:

$ rake docs:generate-docs
NameError: uninitialized constant LogStash::Environment::LOGSTASH_HOME
  const_missing at org/jruby/RubyModule.java:2745
       generate at docs/asciidocgen.rb:164
         (root) at docs/asciidocgen.rb:263
           each at org/jruby/RubyArray.java:1613
         (root) at docs/asciidocgen.rb:261

Is any additional configuration needed to run this rake target? Setting the LOGSTASH_HOME environment variable to the root directory of the checked-out Logstash git doesn't make a difference.