Initialize copy constructor of a derived class from std::string

I have a problem when I derive a String class from std::string and I test it.

Here is the String class:

namespace Types
{

class String : public std::string
{
public:
    String();

    /*!
     * @fn String(const String &other);
     * @brief Copy contructor
     * @param other
     */
    String(const String &other)
        : std::string(other) {}
    String(const char* format_string, ...);

    virtual ~String();
};

} /* namespace Types */

And here is the test class :

class String_test : public CppUnit::TestFixture
{
    CPPUNIT_TEST_SUITE(String_test);
    CPPUNIT_TEST(testCopyConstructor);
    CPPUNIT_TEST_SUITE_END();

public:
    void setUp(void)
    {
        mTestObj = new Types::String();
    }
    void tearDown(void)
    {
        delete mTestObj;
    }

protected:
    void testCopyConstructor(void)
    {
        *mTestObj = "toto";
        std::cout << *mTestObj << std::endl;
        Types::String new_string((const Types::String&)mTestObj);
        CPPUNIT_ASSERT(true == true);
    }
private:
    Types::String *mTestObj;
};

The compilation is ok but when the program is running I have this error:

##Failure Location unknown## : Error
Test name: String_test::testCopyConstructor
uncaught exception of type std::exception (or derived).
- basic_string::_M_construct null not valid

I have searched the documentation about copy constructor and derived class, it seems that is ok in code. I don't understand what's the problem.

Anyone have an idea ?

Thanks