diff -u b/includes/database/mysql/schema.inc b/includes/database/mysql/schema.inc
--- b/includes/database/mysql/schema.inc
+++ b/includes/database/mysql/schema.inc
@@ -150,8 +150,7 @@
       // innodb_large_prefix is not enabled on the server then length should be
       // 191 and not 255 to prevent error
       if ($spec['length'] >= 192 && $info['charset'] == 'utf8mb4') {
-        $spec['length'] = 191;
-        drupal_set_message(t(':name has been truncted to 191 characters because utf8mb4 has been enabled while \'innodb_large_prefix\' is <em>OFF</em>.', array(':name' => $name)), 'warning', FALSE);
+        throw new Exception(t("If you're going to set the charset to UTF8MB4 you must set innodb_large_prefix=ON in your my.cnf config file."));
       }
     }
     if (in_array($spec['mysql_type'], array('VARCHAR', 'CHAR', 'TINYTEXT', 'MEDIUMTEXT', 'LONGTEXT', 'TEXT'))) {