diff --git a/tests/src/Functional/FillPdfFormFormTest.php b/tests/src/Functional/FillPdfFormFormTest.php
index 13531cb0812c01a6772182fb8a6385fe62db4d84..13819775436a3a7c05e8c3e0a4e939c1157d03f2 100644
--- a/tests/src/Functional/FillPdfFormFormTest.php
+++ b/tests/src/Functional/FillPdfFormFormTest.php
@@ -55,7 +55,7 @@ class FillPdfFormFormTest extends FillPdfUploadTestBase {
       $this->assertSession()->fieldValueEquals('default_entity_type', $type);
 
       // Check the default entity ID field is present but empty.
-      $this->assertSession()->fieldValueEquals('default_entity_id', NULL);
+      $this->assertSession()->fieldValueEquals('default_entity_id', '');
 
       if ($count == 0) {
         $options = $this->assertSession()->selectExists('default_entity_id')->findAll('xpath', 'option');
diff --git a/tests/src/Functional/FillPdfSettingsFormTest.php b/tests/src/Functional/FillPdfSettingsFormTest.php
index 561657079e85f5efe366fc9332e1001eaf17c168..a29705f4c290a6a55f44b0da44450d20be4cba6e 100644
--- a/tests/src/Functional/FillPdfSettingsFormTest.php
+++ b/tests/src/Functional/FillPdfSettingsFormTest.php
@@ -254,7 +254,7 @@ class FillPdfSettingsFormTest extends BrowserTestBase {
     $this->assertSession()->pageTextContains('The configuration options have been saved.');
     // So the new values should be submitted *and* saved this time, except for
     // the unrelated 'fillpdf_service_api_key' which should be dismissed.
-    $expected = ['fillpdf_service_api_key' => NULL] + $edit;
+    $expected = ['fillpdf_service_api_key' => ''] + $edit;
     foreach ($expected as $field => $value) {
       $this->assertSession()->fieldValueEquals($field, $value);
       $config_name = $field === 'example_setting' ? 'fillpdf_test.settings' : 'fillpdf.settings';
diff --git a/tests/src/Functional/PdfWebformPopulationTest.php b/tests/src/Functional/PdfWebformPopulationTest.php
index 4cc1547a7a52622b3c876d4798ef6136c18f558c..79515fbe7c96e9cc69d3c0f0c3fea3fba85353d3 100644
--- a/tests/src/Functional/PdfWebformPopulationTest.php
+++ b/tests/src/Functional/PdfWebformPopulationTest.php
@@ -59,7 +59,7 @@ class PdfWebformPopulationTest extends FillPdfTestBase {
     $contact_form = Webform::load('fillpdf_contact');
     $contact_form_test_route = Url::fromRoute('entity.webform.test_form', ['webform' => $contact_form->id()]);
     $this->drupalGet($contact_form_test_route);
-    $this->submitForm([], $this->t('Send message'));
+    $this->submitForm([], (string) $this->t('Send message'));
 
     // Load the submission.
     $this->testSubmission = WebformSubmission::load($this->getLastSubmissionId($contact_form));
diff --git a/tests/src/Traits/TestFillPdfTrait.php b/tests/src/Traits/TestFillPdfTrait.php
index 01adede7750ad6c4c2aceda2e21d1a024fd799be..7401d01e4230080c12ab2643bfa5446419e83557 100644
--- a/tests/src/Traits/TestFillPdfTrait.php
+++ b/tests/src/Traits/TestFillPdfTrait.php
@@ -64,7 +64,7 @@ trait TestFillPdfTrait {
       'local_service_endpoint' => $localServerEndpoint ?: 'http://127.0.0.1:8085',
     ];
     $this->drupalGet('admin/config/media/fillpdf');
-    $this->submitForm($edit, t('Save configuration'));
+    $this->submitForm($edit, (string) t('Save configuration'));
   }
 
   /**