diff --git a/src/Element/WebformRadiosOther.php b/src/Element/WebformRadiosOther.php
index 805b593af..3360aed5f 100644
--- a/src/Element/WebformRadiosOther.php
+++ b/src/Element/WebformRadiosOther.php
@@ -2,6 +2,8 @@
 
 namespace Drupal\webform\Element;
 
+use Drupal\Core\Form\FormStateInterface;
+
 /**
  * Provides a webform element for radio buttons with an other option.
  *
@@ -14,4 +16,27 @@ class WebformRadiosOther extends WebformOtherBase {
    */
   protected static $type = 'radios';
 
+  /**
+   * {@inheritdoc}
+   */
+  public static function valueCallback(&$element, $input, FormStateInterface $form_state) {
+    // Remove 'webform_' prefix from type.
+    $type = str_replace('webform_', '', static::$type);
+
+    // Unset #default_value that is not a valid #option.
+    //
+    // This behavior is needed when a radios #default_value was previously set
+    // but now the radios are unchecked via conditional logic. This results in
+    // nothing being posted back to the server, and the #default_value is used
+    // which throws "An illegal choice has been detected." error.
+    if ($input
+      && !isset($input[$type])
+      && isset($element['#default_value'])
+      && !isset($element['#option'][$element['#default_value']])) {
+      unset($element['#default_value']);
+    }
+
+    return parent::valueCallback($element, $input, $form_state);
+  }
+
 }