problem with (use of) std::vector

SYNOPSIS
=========
I am experiencing a strange problem, which I'm sure is caused by an
oversight on my part - but despite peering over the code for a few
hours, I have been able to spot the bug. I am writing a simple generic
value container. A generic value is implemented as a discriminated union
class (B), which is nested within the container class A. Class A stores
instances of generic values in a std::vector.

The container as described, works well for fundamental data types,
however, when I attempt to store pointer types (which are managed by
class B), a memory access/violation exception is thrown).

SAMPLE CODE TO REPRODUCE ISSUE
===============================
class A
{
public:
    A(){}
    ~A(){}

    size_t addValue(const std::string& s)
    {
        m_values.push_back(B(s.c_str()));
        return m_values.size();
    }

    size_t addValue(const long l)
    {
        m_values.push_back(B(l));
        return m_values.size();
    }

private:
    class B
    {
    public:
        friend class A;

        B():m_stype(false)
        {
            m_value.l = 0;
        }

        explicit B(const long l):m_stype(false)
        {
            m_value.l = l;
        }

        explicit B(const char* s):m_stype(true)
        {
            //error checking omitted for brevity
            m_value.s = strdup(s);
        }

        ~B()
        {
            if (m_stype && m_value.s)
                free(m_value.s);
        }

        B(const B& b):m_stype(b.m_stype), m_value(b.m_value)
        {}

        B& operator= (const B& b)
        {
            if (this != &b)
            {
                //free any allocated resources first
                if (m_stype && m_value.s)
                {
                    free(m_value.s);
                    m_value.s = 0;
                }

                if (b.m_stype && b.m_value.s)
                    m_value.s = strdup(b.m_value.s);
                else
                    m_value.l = b.m_value.l;

                m_stype = b.m_stype;
            }
            return *this;
        }

    private:
        union C
        {
            long l;
            char* s;
        }m_value;
        bool m_stype ;
    };

    std::vector<B> m_values ;
};

int main(int argc, char* argv[])
{
    A a;
    a.addValue(100);
    a.addValue("Hello"); //<- runtime error here
}

Can anyone spot the cause of this error?