diff --git a/lib/form/select.php b/lib/form/select.php
index <HASH>..<HASH> 100644
--- a/lib/form/select.php
+++ b/lib/form/select.php
@@ -71,13 +71,19 @@ class MoodleQuickForm_select extends HTML_QuickForm_select{
      * @return string
      */
     function toHtml(){
+        $html = '';
+        if ($this->getMultiple()) {
+            // Adding an hidden field forces the browser to send an empty data even though the user did not
+            // select any element. This value will be cleaned up in self::exportValue() as it will not be part
+            // of the select options.
+            $html .= '<input type="hidden" name="'.$this->getName().'" value="_qf__force_multiselect_submission">';
+        }
         if ($this->_hiddenLabel){
             $this->_generateId();
-            return '<label class="accesshide" for="'.$this->getAttribute('id').'" >'.
-                        $this->getLabel().'</label>'.parent::toHtml();
-        } else {
-             return parent::toHtml();
+            $html .= '<label class="accesshide" for="'.$this->getAttribute('id').'" >'.$this->getLabel().'</label>';
         }
+        $html .= parent::toHtml();
+        return $html;
     }
 
     /**
MDL-<I> form: Mutiple select work correctly without any selection