diff --git a/core/modules/simpletest/drupal_web_test_case.php b/core/modules/simpletest/drupal_web_test_case.php
index ca66143..bea803b 100644
--- a/core/modules/simpletest/drupal_web_test_case.php
+++ b/core/modules/simpletest/drupal_web_test_case.php
@@ -1459,10 +1459,9 @@ class DrupalWebTestCase extends DrupalTestCase {
     // Delete temporary files directory.
     file_unmanaged_delete_recursive($this->originalFileDirectory . '/simpletest/' . substr($this->databasePrefix, 10));
 
-    // Remove all prefixed tables (all the tables in the schema).
-    $schema = drupal_get_schema(NULL, TRUE);
-    foreach ($schema as $name => $table) {
-      db_drop_table($name);
+    // Remove all prefixed tables.
+    foreach (db_find_tables('%' . $this->databasePrefix . '%') as $table) {
+      db_drop_table(preg_replace('/^.*' . $this->databasePrefix . '(.*)$/', '$1', $table));
     }
 
     // Get back to the original connection.
diff --git a/core/modules/simpletest/simpletest.module b/core/modules/simpletest/simpletest.module
index eb33d1b..c71ee8a 100644
--- a/core/modules/simpletest/simpletest.module
+++ b/core/modules/simpletest/simpletest.module
@@ -428,13 +428,12 @@ function simpletest_clean_environment() {
  * Removed prefixed tables from the database that are left over from crashed tests.
  */
 function simpletest_clean_database() {
-  $tables = db_find_tables(Database::getConnection()->prefixTables('{simpletest}') . '%');
-  $schema = drupal_get_schema_unprocessed('simpletest');
   $count = 0;
-  foreach (array_diff_key($tables, $schema) as $table) {
-    // Strip the prefix and skip tables without digits following "simpletest",
-    // e.g. {simpletest_test_id}.
+  foreach (db_find_tables('%simpletest%') as $table) {
     if (preg_match('/simpletest\d+.*/', $table, $matches)) {
+      // The table names returned by db_find_tables() are prefixed. If we don't
+      // strip the prefix, we end up with 'prefix_prefix_tablename' queries in
+      // db_drop_table(). Hence we use $matches[0] instead of $table.
       db_drop_table($matches[0]);
       $count++;
     }