SqlParameter with default value set to 0 doesn't work as expected

I was doing something like this:

SqlParameter param = new SqlParameter("@Param", 0) { SqlDbType = SqlDbType.Int };

private void TestParam(SqlParameter param) {
   string test = param.Value.ToString();  // Getting NullReferenceException here
}

But I stop getting the exception when I put it like this:

SqlParameter param = new SqlParameter("@Param", SqlDbType.Int)  { Value = 0 };

private void TestParam(SqlParameter param) {
    string test = param.Value.ToString();  // Everything OK
}

Can anyone tell me why SqlParameter assumes 0 is the same as null?

Edit: MSDN Explains this here: SqlParameter Constructor