Paul Mitchum
--- a/core/modules/update/src/UpdateSettingsForm.php
+++ b/core/modules/update/src/UpdateSettingsForm.php
  public function buildForm(array $form, FormStateInterface $form_state) {
    $config = $this->config('update.settings');

+    // Adjust existing value to fit our set of choices.
+    $frequency = $config->get('check.interval_days');
+    if ($frequency < 1) {
+      $frequency = 1;
+    }
+    if ($frequency > 1) {
+      $frequency = 7;
+    }
--- a/core/modules/update/tests/src/Functional/UpdateSettingsFormTest.php 0 → 100644
+++ b/core/modules/update/tests/src/Functional/UpdateSettingsFormTest.php 0 → 100644
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['update'];
+
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
+  /**
+   * Tests different frequency form elements for different values.
+   */
+  public function testFrequencyFormElement() {
+    $this->drupalLogin(
--- a/core/modules/update/src/UpdateSettingsForm.php
+++ b/core/modules/update/src/UpdateSettingsForm.php
      '#default_value' => $config->get('check.disabled_extensions'),
    ];

-    $notification_emails = $config->get('notification.emails');
+    // Empty config is returned as string, not sequence.
+    $notification_emails = $config->get('notification.emails') ?? [];
--- a/core/modules/update/update.install
+++ b/core/modules/update/update.install
      $requirements['update_core']['reason'] = UpdateFetcherInterface::UNKNOWN;
      $requirements['update_core']['description'] = _update_no_data();
    }
+
+    // Report configuration error.
+    $frequency = \Drupal::config('update.settings')->get('check.interval_days');
+    if ($frequency < 1) {
+      $requirements['update_core'] = [
+        'title' => t('Update misconfigured'),
+        'description' => t('Update module is set to check for updates more frequently than once per day. This results in a check every cron run, which is wasteful. <a href=":freq_config">Modify update frequency</a>.', [
+          ':freq_config' => Url::fromRoute('update.settings')->toString(),
+        ]),
+        'severity' => REQUIREMENT_WARNING,
+      ];
+    }
--- a/core/modules/update/update.module
+++ b/core/modules/update/update.module
 function update_cron() {
  $update_config = \Drupal::config('update.settings');
  $frequency = $update_config->get('check.interval_days');
+  if ($frequency < 1) {
+    \Drupal::logger('update')->notice('Update module frequency set to ' . $frequency . ' days. Acting as if it is set to 1. See: [change record]');
+    $frequency = 1;
+  }
--- a/core/modules/update/tests/src/Kernel/UpdateSettingsFormTest.php 0 → 100644
+++ b/core/modules/update/tests/src/Kernel/UpdateSettingsFormTest.php 0 → 100644
+      // Value, expected, form element type.
+      [0, 1, 'radios'],
+      [1, 1, 'radios'],
+      [2, 2, 'number'],
+      [6, 6, 'number'],
+      [7, 7, 'radios'],
+      [8, 8, 'number'],
+    ];
+  }
+
+  /**
+   * @dataProvider providerFormData
+   */
+  public function testUpdateSettingsForm($config_value, $expected, $form_element_type) {
+    // Set config.
+    $config = $this->config('update.settings');
--- a/core/modules/update/src/UpdateSettingsForm.php
+++ b/core/modules/update/src/UpdateSettingsForm.php
+    // Adjust the frequency so it's never 0.
+    $frequency = intval($config->get('check.interval_days'));
+    if ($frequency < 1) {
+      $frequency = 1;
+    }
+    // Use the radio buttons if frequency is daily or weekly.
    $form['update_check_frequency'] = [
      '#type' => 'radios',
      '#title' => $this->t('Check for updates'),
-      '#default_value' => $config->get('check.interval_days'),
+      '#default_value' => $frequency,
      '#options' => [
        '1' => $this->t('Daily'),
        '7' => $this->t('Weekly'),
      ],
      '#description' => $this->t('Select how frequently you want to automatically check for new releases of your currently installed modules and themes.'),