AjaxのPostでTypeError: 'caller', 'callee', and 'arguments' properties may not be accessedが返ってくる

##概要
AjaxのPostメソッドでレスポンスが400エラーになり下記エラーメッセージが返ってきます。
※getメソッドは問題なく動きます。
対処法を教えて欲しいです。

TypeError: 'caller', 'callee', and 'arguments' properties may not be accessed on strict mode functions or the arguments objects for calls to them
    at Function.invokeGetter (<anonymous>:1:142)

##ソースコード

PHP
1function getTodayCount() {
2  global $wpdb;
3  $request = $_GET['today'];
4  $now = date("Y/m/d");
5  $sql = $wpdb->prepare('select count(id) from $wpdb->wp_db_counts where regist_day = %s',$now);
6  $results = $wpdb->get_results($sql);
7  $countResult = $wpdb->num_rows;
8  // header('Content-Type: application/json; charset=UTF-8');
9  // var_dump($countResult);
10  // die();
11  echo $countResult;
12  // echo "test";
13  die();
14}
15add_action('wp_ajax_get_today_count', 'getTodayCount');
16add_action('wp_ajax_nopriv_get_today_count', 'getTodayCount');
17
18// カウントの登録
19function setCount() {
20  global $wpdb;
21  $request = $_POST['today'];
22  $nowDate = date("Y/m/d");
23  $nowTime = date("Y/m/d H:i:s");
24  $table_name = $wpdb->prefix ."db_count";
25  $results = $wpdb->query("insert into ".$table_name . "(regist_day,created_at) values(". $nowDate ."," . $nowTime . ")");
26  if ($results == false) {
27    # code...
28    echo "登録に失敗しました";
29  } else {
30    echo "登録に成功しました";
31  }
32  die();
33}
34add_action('wp_ajax_set_count', 'setCount');
35add_action('wp_ajax_nopriv_set_count', 'setCount');
36

js
1'use strict';
2
3$(function() {
4  var maxCount = 3
5  var ajaxurl = '//localhost:8000/wp-admin/admin-ajax.php';
6  var todayForm = new Date();
7  var y = todayForm.getFullYear();
8  var m = ("00" + (todayForm.getMonth() + 1)).slice(-2);
9  var d = ("00" + todayForm.getDate()).slice(-2);
10  var requestDateForm = y + '/' + m + '/' + d;
11  $.ajax({
12    method: 'GET',
13    url: ajaxurl,
14    // processData: false,
15    contentType: 'application/json',
16    data : {
17      'action': 'get_today_count',
18    },
19  }).then(function (response) {
20    console.log(response);
21    var todayCount = maxCount - parseInt(response);
22    $(".count").text(todayCount);
23  }).catch(function (error) {
24    console.log(error);
25  });
26
27  $("#confirmbtn").on("click", function() {
28    event.preventDefault();
29
30    var fd = new FormData();
31    fd.append('action', 'ajaxtest');
32
33    // ajax通信
34    $.ajax({
35      type: 'POST',
36      url: ajaxurl,
37      // data: {
38      //   'action': 'set_count',
39      // },
40      data : fd,
41      processData: false,
42      contentType : false,
43      // contentType: 'application/json',
44    // 成功時
45    }).then(function (response) {
46      console.log(response);
47    // 失敗時
48    }).catch(function (error) {
49      console.log(error);
50    });
51  })
52});