default variable value doesn't work with other variables

[myazinn]

JsonLogic:

{ "var": [ "a", { "var": "b" } ] }

Data:

{"b": 123}

As Java:

String expression = "{ \"var\": [\"a\", { \"var\": \"b\" }] }";

Map<String, Integer> data = new HashMap<>();
data.put("b", 123);

System.out.println(new JsonLogic().apply(expression, data));

Expected:
123
Actual:
null