How to use Moq object correctly when object is parameter in method

I am not completely new to unit testing, however I am new to the Moq library and I have run into a problem. I am confused on why my unit test fails. Here is the unit test I am trying to write.

[TestMethod]
    public void TestFunction()
    {
        // Arrange
        var options = new Dictionary<string, string>() { { keyValue, true.ToString() } };
        //optionsMock.Setup(a => a.ContainsKey(It.Is<string>(b => b == keyValue))).Returns(false);
        //optionsMock.Setup(c => c.Add(It.Is<string>(d => d == keyValue), It.Is<string>(e => e == true.ToString()))).Verifiable();
        //optionsMock.Setup(f => f[It.Is<string>(g => g == keyValue)]).Returns(true.ToString());

        // Act
        int projectId = sut.Open(stringValue, booleanValue, stringValue, stringValue, IDictionary<string, string>, out errorString);

        // Assert
        //optionsMock.Verify(a => a.ContainsKey(It.Is<string>(b => b == keyValue)), Times.Exactly(2));
        //optionsMock.Verify(c => c.Add(It.Is<string>(d => d == keyValue), It.Is<string>(e => e == true.ToString())), Times.Once());
        //optionsMock.Verify(f => f[It.Is<string>(g => g == keyValue)], Times.Once());
        Assert.AreNotEqual(0, id);
    }

The problem I am experiencing is that eventually sut.Open(...) makes a call to an internal class that has an IDictionary as a parameter. In that method there is a check to see if the dictionary is null. Whenever I run the unit test without a real dictionary, the null check always evaluates to true and I get a false fail. However if the real dictionary is passed in, I get a passing test. Here is an example of the internal code.

public int Open(..., IDictionary<string, string>, ...)
    {
        //...
        InternalClass.Method(IDictionary<string, string>);
        //...
    }

Here is the internal classes method

public void Open(..., IDictionary<string, string> dictionary, ...)
    {
       if(dictionary != null)
       {
          //... Do something
       }
       else
       {
          //... Do something else
       }
    }

It always executes Do something else, is there away around this?