diff --git a/core/modules/simpletest/src/AssertContentTrait.php b/core/modules/simpletest/src/AssertContentTrait.php
index e19f319..c6f8695 100644
--- a/core/modules/simpletest/src/AssertContentTrait.php
+++ b/core/modules/simpletest/src/AssertContentTrait.php
@@ -918,6 +918,10 @@ protected function assertFieldsByValue($fields, $value = NULL, $message = '', $g
       $found = FALSE;
       if ($fields) {
         foreach ($fields as $field) {
+          if (!($field instanceof \SimpleXmlElement)) {
+            throw new \InvalidArgumentException('Wrong trait in WTB!?!');
+          }
+
           if (isset($field['value']) && $field['value'] == $value) {
             // Input element with correct value.
             $found = TRUE;
diff --git a/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php b/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
index 4426d62..1f33959 100644
--- a/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
+++ b/core/tests/Drupal/FunctionalTests/AssertLegacyTrait.php
@@ -569,7 +569,7 @@ protected function assertNoFieldByXPath($xpath, $value = NULL, $message = '') {
   /**
    * Asserts that a field exists in the current page with a given Xpath result.
    *
-   * @param \Behat\Mink\Element\NodeElement[] $fields
+   * @param [] $fields
    *   Xml elements.
    * @param string $value
    *   (optional) Value of the field to assert. You may pass in NULL (default) to skip
@@ -589,6 +589,10 @@ protected function assertFieldsByValue($fields, $value = NULL, $message = '') {
       $found = FALSE;
       if ($fields) {
         foreach ($fields as $field) {
+          if (!($field instanceof \Behat\Mink\Element\NodeElement)) {
+            throw new \InvalidArgumentException('Wrong trait in BTB!?!');
+          }
+
           if ($field->getAttribute('value') == $value) {
             // Input element with correct value.
             $found = TRUE;