warning: offset of on non-standard-layout type 'DerivedClass'

I need to get rid of this warning. As far as I understand, it appears because DerivedClass is not considered as Plain Old Data. I read cppreference about POD and Standard Layout Type but I still do not understand why DerivedClass is not a POD. If I put members of BaseClass into DerivedClass and do not use inheritance - then everything is OK. I use C++11

Here is an example:

#include <iostream>

using namespace std;

class BaseClass
{
public:
    int a;
};

class DerivedClass : public BaseClass
{
public:
    int b;
};

int main(int argc, char *argv[])
{
    // warning: offset of on non-standard-layout type 'DerivedClass'
    int offset = offsetof(DerivedClass, b);

    cout << offset;
}

I appreciate any help.