dynamodb Java Update Item

Table table = dynamoDB.getTable("Table Name");
Map<String, String> consultee = new HashMap<>();
UpdateItemSpec updateItemSpec = new UpdateItemSpec().withPrimaryKey("eventId",eventId)
        .withUpdateExpression("set #consultees = list_append(if_not_exists(consultees, :empty_consultees), :consultees) , #url = :url")
        .withNameMap(new NameMap().with("#consultees", "consultees").with("#url","url"))
        .withValueMap(new ValueMap().withList(":consultees", Arrays.asList(consultee)).withList(":empty_consultees", Arrays.asList()).withString(":url",url)).withReturnValues(ReturnValue.ALL_NEW);

UpdateItemOutcome outcome = table.updateItem(updateItemSpec);