diff --git a/ChangeLog b/ChangeLog
index ac0e6dc380..6547d1ed33 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,9 @@
+Sat May  1 00:26:31 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* lib/test/unit/assertions.rb (Test::Unit::Assertions#assert):
+	  assertion message must be String or Proc.  suggested by caleb
+	  clausen at [ruby-core:29884].
+
 Sat May  1 00:14:47 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* test/ruby/test_io.rb: skip false tests on Windows.
diff --git a/lib/test/unit/assertions.rb b/lib/test/unit/assertions.rb
index 821faf5803..9a6d643623 100644
--- a/lib/test/unit/assertions.rb
+++ b/lib/test/unit/assertions.rb
@@ -10,6 +10,15 @@ module Test
         obj.pretty_inspect.chomp
       end
 
+      def assert(test, msg = (nomsg = true; nil))
+        unless nomsg or msg.instance_of?(String) or msg.instance_of?(Proc) or
+            (bt = caller).shift.rindex(MiniTest::MINI_DIR, 0)
+          bt = MiniTest.filter_backtrace(bt)
+          raise ArgumentError, "assertion message must be String or Proc, but #{msg.class} was given.", bt
+        end
+        super
+      end
+
       def assert_raise(*args, &b)
         assert_raises(*args, &b)
       end
diff --git a/test/testunit/test_assertion.rb b/test/testunit/test_assertion.rb
new file mode 100644
index 0000000000..374d6e022b
--- /dev/null
+++ b/test/testunit/test_assertion.rb
@@ -0,0 +1,8 @@
+require 'test/unit'
+class TestAssertion < Test::Unit::TestCase
+  def test_wrong_assertion
+    error, line = assert_raise(ArgumentError) {assert(true, true)}, __LINE__
+    assert_match(/assertion message must be String or Proc, but TrueClass was given/, error.message)
+    assert_match(/\A#{Regexp.quote(__FILE__)}:#{line}:/, error.backtrace[0])
+  end
+end