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
@@ -9,7 +9,6 @@
 
 use Drupal\Core\Database\Database;
 use Drupal\Core\Database\Query\Condition;
-use Drupal\Core\Database\SchemaException;
 use Drupal\Core\Database\SchemaObjectExistsException;
 use Drupal\Core\Database\SchemaObjectDoesNotExistException;
 use Drupal\Core\Database\Schema as DatabaseSchema;
@@ -305,9 +304,6 @@
    *
    * @return array
    *   List of shortened indexes.
-   *
-   * @throws \Drupal\Core\Database\SchemaException
-   *   Thrown if field specification is missing.
    */
   protected function getNormalizedIndexes(array $spec) {
     $indexes = isset($spec['indexes']) ? $spec['indexes'] : [];
@@ -315,17 +311,15 @@
       foreach ($index_fields as $index_key => $index_field) {
         // Get the name of the field from the index specification.
         $field_name = is_array($index_field) ? $index_field[0] : $index_field;
-        // Check whether the field is defined in the table specification.
+        // Assert that the field is defined in the table specification.
         assert('isset($spec[\'fields\'][$field_name])', "MySQL needs the '$field_name' field specification in order to normalize the '$index_name' index");
-        if (isset($spec['fields'][$field_name])) {
-          // Get the MySQL type from the processed field.
-          $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)) {
-              // Limit the index length to 191 characters.
-              $this->shortenIndex($indexes[$index_name][$index_key]);
-            }
+        // Get the MySQL type from the processed field.
+        $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)) {
+            // Limit the index length to 191 characters.
+            $this->shortenIndex($indexes[$index_name][$index_key]);
           }
         }
       }
only in patch2:
unchanged:
--- a/core/modules/system/src/Tests/Database/SchemaTest.php
+++ b/core/modules/system/src/Tests/Database/SchemaTest.php
@@ -298,18 +298,6 @@ function testIndexLength() {
 
     $schema_object = Database::getConnection()->schema();
 
-    // Ensure expected exception thrown when adding index with missing info.
-    $expected_exception_message = "MySQL needs the 'test_field_text' field specification in order to normalize the 'test_regular' index";
-    $missing_field_spec = $table_specification;
-    unset($missing_field_spec['fields']['test_field_text']);
-    try {
-      $schema_object->addIndex('test_table_index_length', 'test_separate', [['test_field_text', 200]], $missing_field_spec);
-      $this->fail('SchemaException not thrown when adding index with missing information.');
-    }
-    catch (SchemaException $e) {
-      $this->assertEqual($expected_exception_message, $e->getMessage());
-    }
-
     // Add a separate index.
     $schema_object->addIndex('test_table_index_length', 'test_separate', [['test_field_text', 200]], $table_specification);
     $table_specification_with_new_index = $table_specification;