Boost.Python: __init__ accepting None argument.

Question

I have a C++ value type wrapped with Boost.Python which has a concept of a NULL value. The relevant parts of the wrapper code appear as follows:

class_<TCurrency> currency( "TCurrency" )
    .def( init<long>() )
    .def( init<const std::string&>() )
    <...>;

Currently, trying to create a NULL instance in Python by passing None to the __init__() method causes the C++ ctor accepting a const string reference to be called with an invalid reference. (&arg == NULL)

Is it possible to trap the case where None is being passed to a constructor and handle it gracefully or at least to throw a meaningful exception before my program crashes?

Using Boost 1.36 and Python 2.6.2.