VS 2013 exception when using C++11 unrestricted unions

Consider this code:

struct TNumeric {
    bool    Negative;
    wstring Integral;
    wstring Fraction;
};
union TValue {
    // The unnamed structs are needed because otherwise the compiler does not accept it...
    bool                Bit;
    struct{ TNumeric    Numeric; };
    struct{ wstring     Text; };
};

TNumeric Numeric;
TNumeric &rNumeric{ Numeric };
rNumeric.Integral = L"";
rNumeric.Integral.push_back( L'X' ); //OK, no problem

TValue Value;
TValue &rValue{ Value };
rValue.Text = L"";
rValue.Text.push_back( L'X' ); //OK, no problem

rValue.Numeric.Integral = L"";
rValue.Numeric.Integral.push_back( L'X' ); // Exception

In release mode there is no problem. When run in debug mode there is an exception at the last statement in method _Adopt of class _Iterator_base12 in xutility: Access violation reading location 0x0000005C.

In _Adopt the code is only run when _ITERATOR_DEBUG_LEVEL == 2. I tried with

#define _ITERATOR_DEBUG_LEVEL 1

added in my main program but it remains defined as 2. Is there a way to disable the check?