initialization of class member in anonymous union

I have observed that the following code segfaults at the line ar.p():

#include <iostream>

class A
{
public:
  virtual void p() { std::cout<<"A!\n"; }
};

class B : public A
{
public:
  void p() { std::cout<<"B!\n"; }
};

struct Param
{
  enum {AA, BB} tag;
  union {
    A a;
    B b;
  };

  Param(const A &p)
    : tag(AA) {a = p;}

  A &get() {
    switch(tag) {
    case AA: return a;
    case BB: return b;
    }
  }
};

int main() {
  A a;
  a.p();
  Param u(a);
  A &ar = u.get();
  ar.p();
}

However, when I change the Param constructor to:

Param(const A &p)
  : tag(AA), a(p) {}

it does not segfault anymore.

I think it has something to do with the way the vtable ptr for union member a is initialized, but I'd like to understand this bug better.

Thanks for your help.