error:
       … while calling the 'genericClosure' builtin
         at /pwd/lang/eval-fail-genericClosure-operator-not-function.nix:1:1:
            1| builtins.genericClosure {
             | ^
            2|   startSet = [ { key = 1; } ];

       … while evaluating the 'operator' attribute passed as argument to builtins.genericClosure

       error: expected a function but found a string: "not a function"