diff --git a/lib/test/unit/assertions.rb b/lib/test/unit/assertions.rb
index 6e4707fc25..ab3dd83aaa 100644
--- a/lib/test/unit/assertions.rb
+++ b/lib/test/unit/assertions.rb
@@ -10,10 +10,12 @@ 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).first.rindex(MiniTest::MINI_DIR, 0)
-          bt.delete_if {|s| s.rindex(MiniTest::MINI_DIR, 0)}
+      UNASSIGNED = Object.new # :nodoc:
+
+      def assert(test, msg = UNASSIGNED)
+        msg = nil if msg == UNASSIGNED
+        unless String === msg or Proc === msg 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
         super