Null pointer dereferencing exception with release mode but not debug mode

I am trying to run the following code snippet and am getting a null-pointer dereferencing exception when compiled with Visual Studio 2013 in release mode with optimisations enabled, but not in debug mode:

__m128 CalculateRadicals( __m128 values )
{
    static std::unordered_map<float, __m128> setResults;

    // Store our epsilon as a vector for quick comparison:
    __declspec(align(16)) float flEps[4] = { 0.0005f, 0.0005f, 0.0005f, 0.0005f };
    __m128 eps = _mm_load_ps( flEps );

    union U {
        __m128 vec;
        float flArray[4];
    };

    U u;
    u.vec = values;

    float flFirstVal = u.flArray[0];
    auto it = setResults.find( flFirstVal );
    if( it != setResults.end( ) )
    {
        return it->second;
    }

    __m128 prev = _mm_sqrt_ps( _mm_add_ps( values, _mm_sqrt_ps( values ) ) );
    __m128 curr = _mm_sqrt_ps( _mm_add_ps( values, prev ) );

    while( _mm_movemask_ps( _mm_cmplt_ps( _mm_sub_ps( curr, prev ), eps ) ) != 0x0F )
    {
        prev = curr;
        curr = _mm_sqrt_ps( _mm_add_ps( values, prev ) );
    }

    setResults.insert( std::make_pair( flFirstVal, curr ) );
    return curr;
}

The null-pointer exception occurs on the penultimate line setResults.insert( std::make_pair( flFirstVal, curr ) ), however I'm not sure what could be causing it; I do not dereference a pointer on this line. Does anyone have any idea what could be causing this?