diff --git a/ChangeLog b/ChangeLog
index 0c382992e8..9a894da9bb 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Fri Oct 22 14:50:13 2010  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	* lib/test/unit/assertions.rb (Test::Unit::Assertions#assert):
+	  assertion message must not be nil.
+
 Fri Oct 22 13:59:50 2010  NARUSE, Yui  <naruse@ruby-lang.org>
 
 	* lib/test/unit/assertions.rb (Test::Unit::Assertions#assert):
diff --git a/lib/test/unit/assertions.rb b/lib/test/unit/assertions.rb
index b9fc2bcf12..ce28042c49 100644
--- a/lib/test/unit/assertions.rb
+++ b/lib/test/unit/assertions.rb
@@ -13,8 +13,11 @@ module Test
       UNASSIGNED = Object.new # :nodoc:
 
       def assert(test, msg = UNASSIGNED)
-        msg = nil if msg == UNASSIGNED
-        unless String === msg or Proc === msg or msg.nil? then
+        case msg
+        when UNASSIGNED
+          msg = nil
+        when String, Proc
+        else
           bt = caller.reject { |s| s.rindex(MiniTest::MINI_DIR, 0) }
           raise ArgumentError, "assertion message must be String or Proc, but #{msg.class} was given.", bt
         end