diff --git a/lib/db/upgrade.php b/lib/db/upgrade.php
index 40be53e8751..06295e35d4b 100644
--- a/lib/db/upgrade.php
+++ b/lib/db/upgrade.php
@@ -101,7 +101,9 @@ function xmldb_main_upgrade($oldversion) {
         $index = new xmldb_index('contextid-lowerboundary', XMLDB_INDEX_NOTUNIQUE, array('contextid', 'lowerboundary'));
 
     /// Launch drop index contextid-lowerboundary
-        $dbman->drop_index($table, $index);
+        if ($dbman->index_exists($table, $index)) {
+            $dbman->drop_index($table, $index);
+        }
 
     /// Define index contextid-lowerboundary-letter (unique) to be added to grade_letters
         $table = new xmldb_table('grade_letters');
diff --git a/lib/ddl/database_manager.php b/lib/ddl/database_manager.php
index 196c2a07aca..0e288fafcf2 100644
--- a/lib/ddl/database_manager.php
+++ b/lib/ddl/database_manager.php
@@ -907,7 +907,7 @@ class database_manager {
         if (!$this->index_exists($xmldb_table, $xmldb_intex)) {
             throw new ddl_exception('ddlunknownerror', null,
                       'Index ' . $xmldb_table->getName() . '->' . $xmldb_intex->getName() .
-                      ' does not exist. Delete skipped');
+                      ' does not exist. Drop skipped');
         }
 
         if (!$sqlarr = $this->generator->getDropIndexSQL($xmldb_table, $xmldb_intex)) {