usr/local/lib/ruby/1.8/rexml/formatters/pretty.rb:131:in `': no implicit conversion from nil to integer (TypeError)
To fix it change the following line in lib/rcov/report.rb of your rcov gem
if RUBY_VERSION == "1.8.6" && defined? REXML::Formatters::Transitive
if ["1.8.7", "1.8.6"].include?(RUBY_VERSION) && defined? REXML::Formatters::Transitive
With that modification rcov runs without problems again.