Mocking ConnectionStrings

I'm trying to mock some ConnectionStrings

Using recorder As RecordExpectations = RecorderManager.StartRecording
recorder.ExpectAndReturn(ConfigurationManager.ConnectionStrings.Item("DB:database"), New System.Configuration.ConnectionStringSettings("DB:DocumentDataControl", "Data Source=ecserver6;Initial Catalog=pqr;User Id=userid;Password=blah;", "System.Data.SqlClient")).RepeatAlways()
End Using

However, I keep getting error. I'm pretty sure configurationmanager is not in the mscorlib. Help!

Error 1 TestCase 'CreateObjects.CreateDocumentDataControlMaster'
failed:
*** Mocked return value of ConnectionStringSettingsCollection.get_Item() is unknown, use recorder.Return().
*** Note: Cannot mock types from mscorlib assembly.
TypeMock.TypeMockException
Message:
*** Mocked return value of ConnectionStringSettingsCollection.get_Item() is unknown, use recorder.Return().
*** Note: Cannot mock types from mscorlib assembly.
Source: TypeMock
StackTrace:
at TypeMock.RecorderManager.d()
at TypeMock.RecorderManager.e()
at TypeMock.RecordExpectations.Dispose()
at Eutech.DocumentControl.BLUnitTests.CreateObjects.CreateDocumentDataControlMaster() in C:WebsitesEutech.Web.Application.DocumentControlEutech.DocumentControl.BLUnitTestsClass1.vb:line 72
at TypeMock.MethodDecorator.CallRealMethod()
at TypeMock.DecoratorAttribute.CallDecoratedMethod()
at TypeMock.ClearMocksAttribute.Execute()
at TypeMock.MethodDecorator.f()
at TypeMock.MockManager.a(String A_0, String A_1, Object A_2, Object A_3, Boolean A_4, Object[] A_5)
at TypeMock.InternalMockManager.getReturn(Object that, String typeName, String methodName, Object methodParameters, Boolean isInjected)