diff -u b/core/modules/system/system.install b/core/modules/system/system.install
--- b/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -1451,7 +1451,7 @@
 /**
  * Drop the 'all' index on the 'key_value_expire' table.
  */
-function system_update_9201() {
+function system_post_update_9201() {
   $schema = \Drupal::database()->schema();
   if ($schema->tableExists('key_value_expire')) {
     $schema->dropIndex('key_value_expire', 'all');
diff -u b/core/modules/system/tests/src/Functional/Update/DropIndexAllOnKeyValueExpireTableUpdateTest.php b/core/modules/system/tests/src/Functional/Update/DropIndexAllOnKeyValueExpireTableUpdateTest.php
--- b/core/modules/system/tests/src/Functional/Update/DropIndexAllOnKeyValueExpireTableUpdateTest.php
+++ b/core/modules/system/tests/src/Functional/Update/DropIndexAllOnKeyValueExpireTableUpdateTest.php
@@ -8,7 +8,7 @@
  * Tests that 'all' index is dropped from the 'key_value_expire' table.
  *
  * @group Update
- * @see system_update_9201
+ * @see system_post_update_9201
  */
 class DropIndexAllOnKeyValueExpireTableUpdateTest extends UpdatePathTestBase {