Friend function is unable to construct a unique pointer of the class

I have a certain design strategy where the constructor of my class is private and can only be constructed by friends of the class. Inside the friend function, I am trying to create a unique_pointer of my class using std::make_uniquebut it doesn't compile. My VC12 compiler complains

c:\program files (x86)\microsoft visual studio 12.0\vc\include\memory(1639): error C2248: 'Spam::Spam' : cannot access private member declared in class 'Spam'

The relevant code which fails during compilation is as follows

#include <memory>
class Spam {
public:
    friend void Foo();

private:
    Spam(int mem) :mem(mem) {}
    int mem;
};
void Foo() {
    std::unique_ptr<Spam> spam = std::make_unique<Spam>(10);
}

Why am I not able to compile?