diff --git a/ChangeLog b/ChangeLog
index a34dbbfe68..0c382992e8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Fri Oct 22 13:59:50 2010  NARUSE, Yui  <naruse@ruby-lang.org>
+
+	* lib/test/unit/assertions.rb (Test::Unit::Assertions#assert):
+	  treat nil case. Please run test-all before commit such change.
+
 Thu Oct 21 23:58:14 2010  Koichi Sasada  <ko1@atdot.net>
 
 	* gc.c (gc_lazy_sweep): Variable declarations should be at
diff --git a/lib/test/unit/assertions.rb b/lib/test/unit/assertions.rb
index ab3dd83aaa..b9fc2bcf12 100644
--- a/lib/test/unit/assertions.rb
+++ b/lib/test/unit/assertions.rb
@@ -14,7 +14,7 @@ module Test
 
       def assert(test, msg = UNASSIGNED)
         msg = nil if msg == UNASSIGNED
-        unless String === msg or Proc === msg then
+        unless String === msg or Proc === msg or msg.nil? then
           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