Failing when using Dir.mktmpdir in spec

[ufo2mstar]

Hi, I am getting the following error when I tried to use the fivemat formatter (Win 7, Ruby 2.2+, Rspec 3.7+)

Failure/Error:
       Dir.mktmpdir do |dir|
...
...
NoMethodError:
       undefined method `mktmpdir' for Dir:Class

The specs run fine with the default formatter, and I am not able to find any namespace conflicts. Any idea, anyone?