I don't know why this warning didn't come up during normal testing...

[git-p4: depot-paths = "//src/minitest/dev/": change = 8176]

lib/minitest/unit.rb

@@ -988,7 +988,7 @@ def initialize # :nodoc:
       @report = []
       @errors = @failures = @skips = 0
       @verbose = false
-      @mutex = Mutex.new if defined?(Mutex)
+      @mutex = defined?(Mutex) ? Mutex.new : nil
     end
 
     def synchronize # :nodoc: