Creating unsigned_set of unsigned_set

I want to create a container that will store unique sets of integers inside.

I want to create something similar to

std::unordered_set<std::unordered_set<unsigned int>>

But g++ does not let me do that and says:

invalid use of incomplete type 'struct std::hash<std::unordered_set<unsigned int> >'

What I want to achieve is to have unique sets of unsigned ints.

How can I do that?