How to declare unique_ptr of vector?

I am trying to declare a global vector of MyClass using unique_ptr.

*glo.h

extern std::unique_ptr<std::vector<MyClass>> gl_vec;

*glo.cpp

std::unique_ptr<std::vector<MyClass>> gl_vec;

And in the file where I initialize and use it for the first time in a different *.cpp file:

#include "glo.h"

gl_vec = std::unique_ptr<std::vector<MyClass>> ();

cout << "gl_vec size = " << (*gl_vec).size() << endl; // crashes here

Things keep crashing when I use the pointer. Anyone see what I'm doing wrong?