diff --git a/library/HTMLPurifier/AttrDef/BackgroundPosition.php b/library/HTMLPurifier/AttrDef/BackgroundPosition.php
index 0c620b39..f808fff5 100644
--- a/library/HTMLPurifier/AttrDef/BackgroundPosition.php
+++ b/library/HTMLPurifier/AttrDef/BackgroundPosition.php
@@ -88,14 +88,14 @@ class HTMLPurifier_AttrDef_BackgroundPosition extends HTMLPurifier_AttrDef
             }
             
             // test for length
-            $r = $this->length->validate($bit, $config, &$context);
+            $r =& $this->length->validate($bit, $config, &$context);
             if ($r !== false) {
                 $measures[] = $r;
                 $i++;
             }
             
             // test for percentage
-            $r = $this->percentage->validate($bit, $config, &$context);
+            $r =& $this->percentage->validate($bit, $config, &$context);
             if ($r !== false) {
                 $measures[] = $r;
                 $i++;