issue #130

gistfile1.txt

diff --git a/lib/minitest/mock.rb b/lib/minitest/mock.rb
index 59c531e..876be20 100644
--- a/lib/minitest/mock.rb
+++ b/lib/minitest/mock.rb
@@ -11,6 +11,7 @@ module MiniTest
 
   class Mock
     alias :__respond_to? :respond_to?
+    alias :__singleton_class :singleton_class
 
     skip_methods = %w(object_id respond_to_missing? inspect === to_s)
 
@@ -45,6 +46,7 @@ module MiniTest
     #   @mock.verify  # => raises MockExpectationError
 
     def expect(name, retval, args=[])
+      __singleton_class.send :undef_method, name if __respond_to?(name, true)
       raise ArgumentError, "args must be an array" unless Array === args
       @expected_calls[name] << { :retval => retval, :args => args }
       self
diff --git a/test/minitest/test_minitest_mock.rb b/test/minitest/test_minitest_mock.rb
index 234bc3f..7bb66f7 100644
--- a/test/minitest/test_minitest_mock.rb
+++ b/test/minitest/test_minitest_mock.rb
@@ -55,6 +55,11 @@ class TestMiniTestMock < MiniTest::Unit::TestCase
     assert_equal "mocked method :sum expects 2 arguments, got 0", e.message
   end
 
+  def test_calling_method_call_on_mock
+    @mock = MiniTest::Mock.new.expect(:call, nil)
+    assert_nil @mock.call
+  end
+
   def test_return_mock_does_not_raise
     retval = MiniTest::Mock.new
     mock = MiniTest::Mock.new