What's worng withe this initialization of unique_ptr?

Can somebody tell me, what is wrong with the following initialization of unique_ptr?

int main()
{
  unique_ptr<int> py(nullptr);
  py = new int;
  ....
}

g++ -O2 xxx.cc -lm -o xxx -std=c++11 says:

error: no match for ‘operator=’ (operand types are    ‘std::unique_ptr<int>’ and ‘int*’)
   py = new int;
      ^

doing

unique_ptr<int> px(new int);

just works fine.

Any hints?