diff -u b/core/includes/install.inc b/core/includes/install.inc
--- b/core/includes/install.inc
+++ b/core/includes/install.inc
@@ -981,16 +981,18 @@
       if (isset($requirement['severity']) && $requirement['severity'] == REQUIREMENT_ERROR) {
         $message = [];
 
-        $message['description'] = [
-          '#markup' => $requirement['description'],
-          '#weight' => 0,
-        ];
-
-        // If the requirement description is actually a render array, correct
-        // the message array to recognize this.
+        // If the requirement description is already a render array, add it
+        // to the message array.
         if (is_array($requirement['description'])) {
           $message['description'] = $requirement['description'];
         }
+        // Otherwise, add a #markup element for the description string.
+        else {
+          $message['description'] = ['#markup' => $requirement['description']];
+        }
+        // Set the description weight to 0. If the description was previously
+        // a render array, we overwrite the existing weight here.
+        $message['description']['#weight'] = 0;
 
         // If a required value was provided for the item, add it to the error
         // message. See the hook_requirements() documentation.
@@ -1002,8 +1004,7 @@
             '#weight' => 1,
           ];
         }
-        \Drupal::service('renderer')->renderPlain($message);
-        drupal_set_message($message, 'error');
+        drupal_set_message(\Drupal::service('renderer')->renderPlain($message), 'error');
       }
     }
     return FALSE;