Why can you assign nullptr to std::string?

So today I wrote a fairly hard to find bug where I initialized a std::string to nullptr (not a pointer to std::string, but the value itself). I've found apparently it's only possible to do in C++11 or later with clang.

#include <string>
#include <iostream>

using namespace std;
class Meh{
    int x;
};
class Foo
{
private:
  std::string x=nullptr;
  Meh y=nullptr; //remove this line and it compiles
public:
  std::string z=nullptr;
};

int main(void)
{
    Foo f;
    cout << f.z;
    return 0;
}

As you can see, I tried assigning nullptr to just a random instance of a class and it didn't work. What magic is in string that allows this to work, and in what way is this even valid syntax? I assumed I would be met with a type casting error in this case.

For reference I compiled with this:

clang++ test.cpp -O3 -g -fno-inline -std=c++11 -Wall

It gave no form of warnings, though it would error out if not using C++11