mspec stricter

diff --git a/lib/mspec/expectations/should.rb b/lib/mspec/expectations/should.rb
index 6a37401..6cd7030 100644
--- a/lib/mspec/expectations/should.rb
+++ b/lib/mspec/expectations/should.rb
@@ -1,8 +1,9 @@
 class Object
-  def should(matcher=nil)
+  NO_MATCHER_GIVEN = Object.new
+  def should(matcher=NO_MATCHER_GIVEN)
     MSpec.expectation
     MSpec.actions :expectation, MSpec.current.state
-    if matcher
+    unless matcher.equal?(NO_MATCHER_GIVEN)
       unless matcher.matches?(self)
         SpecExpectation.fail_with(*matcher.failure_message)
       end
@@ -11,10 +12,10 @@ class Object
     end
   end
 
-  def should_not(matcher=nil)
+  def should_not(matcher=NO_MATCHER_GIVEN)
     MSpec.expectation
     MSpec.actions :expectation, MSpec.current.state
-    if matcher
+    unless matcher.equal?(NO_MATCHER_GIVEN)
       if matcher.matches?(self)
         SpecExpectation.fail_with(*matcher.negative_failure_message)
       end