diff -u b/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
--- b/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -1134,11 +1134,11 @@
     return;
   }
 
-  // In PHP 7, just make sure assert failures are thrown.
+  // In PHP 7, we just make sure assert failures are thrown as exceptions.
   if (version_compare(PHP_VERSION, '7.0.0-dev') >= 0) {
     assert_options(ASSERT_EXCEPTION, 1);
   }
-  // In PHP 5, and establish an assert callback handle that will throw AssertionErrors.
+  // In PHP 5, we establish an assert callback handle that will throw AssertionErrors.
   elseif (assert_options(ASSERT_CALLBACK) === NULL) {
     class AssertionError extends Exception {
       public function __construct($message, $code = 0, Exception $previous = NULL, $file, $line) {
@@ -1150,6 +1150,7 @@
     }
 
     assert_options(ASSERT_CALLBACK, function($file, $line, $code, $message = '') {
+      // Use the same message as PHP 7 AssertionErrors.
       if (empty($message)) {
         $message = $code;
       }