string in C# and String^ in C++/CLI... How to pass "null"?

I got some C++ library. And also I got C++/CLI wrapper for it, to make it possible call methods from this library in C# code.

Lets say I would like to use some call in C# like this:

string date = MyWrapper.GetValue("SystemSettings", "BuildDate", null);

which will call next function on C++/CLI:

static String^ GetValue(String^ section, String^ key, String^ defaultValue)

My problem: I got next ArgumentNullException:

Value cannot be null.\r\nParameter name: managedString.

So... Question: how should I pass null correctly? Thanks.