diff -u b/core/lib/Drupal/Core/Database/Driver/mysql/Schema.php b/core/lib/Drupal/Core/Database/Driver/mysql/Schema.php
--- b/core/lib/Drupal/Core/Database/Driver/mysql/Schema.php
+++ b/core/lib/Drupal/Core/Database/Driver/mysql/Schema.php
@@ -142,9 +142,9 @@
    * Before passing a field out of a schema definition into this function it has
    * to be processed by _db_process_field().
    *
-   * @param $name
+   * @param string $name
    *   Name of the field.
-   * @param $spec
+   * @param array $spec
    *   The field specification, as per the schema data structure format.
    */
   protected function createFieldSql($name, $spec) {
@@ -317,7 +317,7 @@
           $mysql_field = $this->processField($spec['fields'][$field_name]);
           if (in_array($mysql_field['mysql_type'], $this->mysqlStringTypes)) {
             // Check whether we need to shorten the index.
-            if (!(isset($mysql_field['type']) && $mysql_field['type'] == 'varchar_ascii') && !(isset($mysql_field['length']) && $mysql_field['length'] <= 191)) {
+            if ((!isset($mysql_field['type']) || $mysql_field['type'] != 'varchar_ascii') && (!isset($mysql_field['length']) || $mysql_field['length'] > 191)) {
               // Limit the index length to 191 characters.
               $this->shortenIndex($indexes[$index_name][$index_key]);
             }
@@ -333,7 +333,7 @@
    *
    * Shortens an index to 191 characters.
    *
-   * @param $index
+   * @param array $index
    *   The index array to be used in createKeySql.
    *
    * @see Drupal\Core\Database\Driver\mysql\Schema::createKeySql()