How to insert elements into shared pointer of std::map?

I can insert with map like this:

std::map<int, int> testMap;
testMap.insert(std::make_pair(0, 1));

But if I surround the map with shared pointer like this:

std::shared_ptr<std::map<int, int>> testSharedMap;
testSharedMap->insert(std::make_pair(0, 1));

It doesn't work. I get a runtime error.

Exception thrown: read access violation.
_Scary was nullptr. occurred

How can I use the std::map when it was surrounded by std::shared_ptr?