diff --git a/core/lib/Drupal/Component/Render/FormattableMarkup.php b/core/lib/Drupal/Component/Render/FormattableMarkup.php
index fe4d5d4..6797b97 100644
--- a/core/lib/Drupal/Component/Render/FormattableMarkup.php
+++ b/core/lib/Drupal/Component/Render/FormattableMarkup.php
@@ -234,7 +234,7 @@ protected static function placeholderFormat($string, array $args) {
             // in the future without breaking backward compatibility.
             trigger_error('Invalid placeholder (' . $key . ') in string: ' . $string, E_USER_ERROR);
           }
-          elseif (strpos($string, $key)) {
+          elseif (strpos($string, $key) !== FALSE) {
             trigger_error('Invalid placeholder (' . $key . ') in string: ' . $string, E_USER_DEPRECATED);
           }
           // No replacement possible therefore we can discard the argument.
diff --git a/core/tests/Drupal/Tests/Component/Render/FormattableMarkupTest.php b/core/tests/Drupal/Tests/Component/Render/FormattableMarkupTest.php
index 2d5ca8c..5d4cad9 100644
--- a/core/tests/Drupal/Tests/Component/Render/FormattableMarkupTest.php
+++ b/core/tests/Drupal/Tests/Component/Render/FormattableMarkupTest.php
@@ -93,6 +93,9 @@ public function providerTestUnexpectedPlaceholder() {
     return [
       ['Non alpha starting character: ~placeholder', ['~placeholder' => 'replaced'], E_USER_ERROR, 'Invalid placeholder (~placeholder) in string: Non alpha starting character: ~placeholder'],
       ['Alpha starting character: placeholder', ['placeholder' => 'replaced'], E_USER_DEPRECATED, 'Invalid placeholder (placeholder) in string: Alpha starting character: placeholder'],
+      // Ensure that where the placeholder is located in the the string is
+      // irrelevant.
+      ['placeholder', ['placeholder' => 'replaced'], E_USER_DEPRECATED, 'Invalid placeholder (placeholder) in string: placeholder'],
     ];
   }