error:
       … while calling the 'genericClosure' builtin
         at /pwd/lang/eval-fail-genericClosure-missing-operator.nix:1:1:
            1| builtins.genericClosure {
             | ^
            2|   startSet = [ { key = 1; } ];

       … in the attrset passed as argument to builtins.genericClosure

       error: attribute 'operator' missing