Rescue TypeError for #free_port?

Running more than one watirspec test at a time gives me this:

/home/titus/workspace/watir-webdriver/spec/watirspec/lib/server.rb:78:in `initialize': no implicit conversion of nil into String (TypeError)
	from /home/titus/workspace/watir-webdriver/spec/watirspec/lib/server.rb:78:in `new'
	from /home/titus/workspace/watir-webdriver/spec/watirspec/lib/server.rb:78:in `free_port?'
	from /home/titus/workspace/watir-webdriver/spec/watirspec/lib/server.rb:55:in `find_free_port_above'
	from /home/titus/workspace/watir-webdriver/spec/watirspec/lib/server.rb:109:in `<class:Server>'
	from /home/titus/workspace/watir-webdriver/spec/watirspec/lib/server.rb:6:in `<module:WatirSpec>'
	from /home/titus/workspace/watir-webdriver/spec/watirspec/lib/server.rb:5:in `<top (required)>'
	from /home/titus/workspace/watir-webdriver/spec/watirspec/spec_helper.rb:11:in `require'
	from /home/titus/workspace/watir-webdriver/spec/watirspec/spec_helper.rb:11:in `<top (required)>'
	from /home/titus/workspace/watir-webdriver/spec/element_spec.rb:1:in `require'
	from /home/titus/workspace/watir-webdriver/spec/element_spec.rb:1:in `<top (required)>'