Why does unittest.mock fail when the production class constructor takes extra arguments?

I have run into a problem which I think might be a bug with the libraries I am using. However, I am fairly new to python, unittest, and unittest.mock libraries so this may just be a hole in my understanding.

While adding tests to some production code I have run into an error, I have produced a minimal sample that reproduces the issue:

import unittest
import mock

class noCtorArg:
    def __init__(self):
        pass
    def okFunc(self):
        raise NotImplemented


class withCtorArg:
    def __init__(self,obj):
        pass
    def notOkFunc(self):
        raise NotImplemented
    def okWithArgFunc(self, anArgForMe):
        raise NotImplemented

class BasicTestSuite(unittest.TestCase):
    """Basic test Cases."""

    # passes
    def test_noCtorArg_okFunc(self):
        mockSUT = mock.MagicMock(spec=noCtorArg)
        mockSUT.okFunc()
        mockSUT.assert_has_calls([mock.call.okFunc()])

    # passes
    def test_withCtorArg_okWithArgFuncTest(self):
        mockSUT = mock.MagicMock(spec=withCtorArg)
        mockSUT.okWithArgFunc("testing")
        mockSUT.assert_has_calls([mock.call.okWithArgFunc("testing")])

    # fails
    def test_withCtorArg_doNotOkFuncTest(self):
        mockSUT = mock.MagicMock(spec=withCtorArg)
        mockSUT.notOkFunc()
        mockSUT.assert_has_calls([mock.call.notOkFunc()])


if __name__ == '__main__':
    unittest.main()

How I run the tests and the output is as follows:

E:work>python -m unittest testCopyFuncWithMock
.F.
======================================================================
FAIL: test_withCtorArg_doNotOkFuncTest (testCopyFuncWithMock.BasicTestSuite)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "testCopyFuncWithMock.py", line 38, in test_withCtorArg_doNotOkFuncTest
    mockSUT.assert_has_calls([mock.call.notOkFunc()])
  File "C:Python27libsite-packagesmockmock.py", line 969, in assert_has_calls
    ), cause)
  File "C:Python27libsite-packagessix.py", line 718, in raise_from
    raise value
AssertionError: Calls not found.
Expected: [call.notOkFunc()]
Actual: [call.notOkFunc()]

----------------------------------------------------------------------
Ran 3 tests in 0.004s

FAILED (failures=1)

I am using python 2.7.11, with mock version 2.0.0 installed via pip.

Any suggestions for what I am doing wrong? Or does this look like a bug in the library?