Update status of parent when all subtasks completed

How do I correctly identify subtasks and check if they are resolved? 
I am getting an error that I am calling “undefined.every” while trying to update an epic to complete when all its children are completed.

var entities = require('@jetbrains/youtrack-scripting-api/entities');

exports.rule = entities.Issue.onChange({
 title: 'Set Epic to Done when all subtasks are resolved',
 guard: function(ctx) {
   return ctx.issue.links["subtask of"].isNotEmpty();
 },
 action: function(ctx) {
   var parent = ctx.issue.links["subtask of"].first();

   var allResolved = parent.links["subtasks"].every(function(subtask) {
     return subtask.isResolved;
   });

   if (allResolved && !parent.isResolved) {
     parent.fields.State = ctx.State.Done;
   }
 },
 requirements: {
   State: {
     type: entities.State.fieldType,
     "Done": {}
   }
 }
});