diff --git a/backup/util/dbops/restore_dbops.class.php b/backup/util/dbops/restore_dbops.class.php
index f56de9be243..9042c4f4fde 100644
--- a/backup/util/dbops/restore_dbops.class.php
+++ b/backup/util/dbops/restore_dbops.class.php
@@ -1811,10 +1811,9 @@ abstract class restore_dbops {
     public static function calculate_course_names($courseid, $fullname, $shortname) {
         global $CFG, $DB;
 
-        $currentfullname = '';
-        $currentshortname = '';
         $counter = 0;
-        // Iteratere while the name exists
+
+        // Iterate while fullname or shortname exist.
         do {
             if ($counter) {
                 $suffixfull  = ' ' . get_string('copyasnoun') . ' ' . $counter;
@@ -1823,8 +1822,11 @@ abstract class restore_dbops {
                 $suffixfull  = '';
                 $suffixshort = '';
             }
-            $currentfullname = $fullname.$suffixfull;
-            $currentshortname = substr($shortname, 0, 100 - strlen($suffixshort)).$suffixshort; // < 100cc
+
+            // Ensure we don't overflow maximum length of name fields, in multi-byte safe manner.
+            $currentfullname = core_text::substr($fullname, 0, 254 - strlen($suffixfull)) . $suffixfull;
+            $currentshortname = core_text::substr($shortname, 0, 100 - strlen($suffixshort)) . $suffixshort;
+
             $coursefull  = $DB->get_record_select('course', 'fullname = ? AND id != ?',
                     array($currentfullname, $courseid), '*', IGNORE_MULTIPLE);
             $courseshort = $DB->get_record_select('course', 'shortname = ? AND id != ?', array($currentshortname, $courseid));