Permission denied - db/test.sqlite3

I'm investigating creating a plugin for Redmine and having a good old time when I decide to clear out the test database:

C:\dev\redmine>rake db:test:purge
(in C:/dev/redmine)
rake aborted!
Permission denied - db/test.sqlite3

Boo! 

I don't know what made me think of it but I tried unsetting my RAILS_ENV environment variable (I had it set to "test"):

C:\dev\redmine>set rails_env=

C:\dev\redmine>rake db:test:purge
(in C:/dev/redmine)

C:\dev\redmine>

Hurrah!