diff --git a/lib/test/unit/assertions.rb b/lib/test/unit/assertions.rb
index 39a3c85600..f810d5a714 100644
--- a/lib/test/unit/assertions.rb
+++ b/lib/test/unit/assertions.rb
@@ -319,6 +319,14 @@ EOT
         template &&= template.chomp
         template.gsub(/\G((?:[^\\]|\\.)*?)(\\)?\?/) { $1 + ($2 ? "?" : mu_pp(arguments.shift)) }
       end
+
+      def message(msg = nil, *args, &default)
+        if Proc === msg
+          super(nil, *args) {"#{msg.call}#{default.call if default}"}
+        else
+          super
+        end
+      end
     end
   end
 end
diff --git a/test/mkmf/base.rb b/test/mkmf/base.rb
index 7df1e8961f..f4f00ed534 100644
--- a/test/mkmf/base.rb
+++ b/test/mkmf/base.rb
@@ -10,9 +10,6 @@ $extout_prefix = "$(extout)$(target_prefix)/"
 
 class TestMkmf < Test::Unit::TestCase
   MKMFLOG = proc {File.read("mkmf.log") rescue ""}
-  class << MKMFLOG
-    alias to_s call
-  end
 
   class Capture
     attr_accessor :origin
@@ -54,11 +51,7 @@ class TestMkmf < Test::Unit::TestCase
   attr_reader :stdout
 
   def mkmflog(msg)
-    log = proc {MKMFLOG[] << msg}
-    class << log
-      alias to_s call
-    end
-    log
+    proc {MKMFLOG[] << msg}
   end
 
   def setup