diff --git a/test/coverage/html_formatter.rb b/test/coverage/html_formatter.rb
index 99b101afa..e8564a816 100644
--- a/test/coverage/html_formatter.rb
+++ b/test/coverage/html_formatter.rb
@@ -24,7 +24,7 @@ module Redmine
     class HtmlFormatter
       def format(result)
         File.open(File.join(output_path, "index.html"), "w") do |file|
-          file.puts template('index').result(binding)
+          file.puts template('index').result(binding).force_encoding('utf-8')
         end
         result.source_files.each do |source_file|
           File.open(File.join(output_path, source_file_result(source_file)), "w") do |file|