What are the causes and fix of Heroku's NoMethodError?

Question

Just last night, I was executing "heroku pg:psql" on my computer, but this morning I am greeted with the following:

H:\MyName\My Documents\MyOrg\Heroku\MyApp>heroku pg:psql
 !    Heroku client internal error.
 !    Search for help at: https://help.heroku.com
 !    Or report a bug at: https://github.com/heroku/heroku/issues/new

    Error:       undefined method `map' for #<String:0x346eae0> (NoMethodError)
    Backtrace:       C:/Users/myname/.heroku/client/lib/heroku/helpers/heroku_postgresql.rb:99:in `app_attachments'
                     C:/Users/myname/.heroku/client/lib/heroku/helpers/heroku_postgresql.rb:104:in `hpg_databases'
                 C:/Users/myname/.heroku/client/lib/heroku/helpers/heroku_postgresql.rb:159:in `hpg_resolve'
                 C:/Users/myname/.heroku/client/lib/heroku/helpers/heroku_postgresql.rb:58:in `resolve'
                 C:/Users/myname/.heroku/client/lib/heroku/command/pg.rb:91:in `psql'
                 C:/Users/myname/.heroku/client/lib/heroku/command.rb:218:in `run'
                 C:/Users/myname/.heroku/client/lib/heroku/cli.rb:37:in `start'
                 C:/Program Files (x86)/Heroku/bin/heroku:29:in `<main>'

    Command:     heroku pg:psql
    Version:     heroku/toolbelt/3.9.5 (i386-mingw32) ruby/1.9.3

Why would this error suddenly arise while I slept, and, moreover, how can I fix it?

Answer 1

Someone broke Heroku Toolbelt, but a fix is in now:

https://status.heroku.com/incidents/650

https://github.com/heroku/heroku/commit/784b1d1439bbd044afd892403a2e63b9114546fb