Using an unordered_map with arrays as keys

I don't understand why I can't have an unordered_map with an array<int,3> as the key type:

#include <unordered_map>

using namespace std;

int main() {

   array<int,3> key = {0,1,2};

   unordered_map< array<int,3> , int >  test;
   test[key]  = 2;

   return 0;
}

I get a long error, the most pertinent part being

main.cpp:11:9: error: no match for ‘operator[]’ (operand types are std::unordered_map<std::array<int, 3ul>, int>’ and ‘std::array<int, 3ul>’)
 test[key]  = 2;
     ^

Are arrays not eligible to be keys because they miss some requirements?