diff --git a/UI/WebServerResources/prototype.js b/UI/WebServerResources/prototype.js
index 37dd39ac2..bdc0f9f23 100644
--- a/UI/WebServerResources/prototype.js
+++ b/UI/WebServerResources/prototype.js
@@ -3157,7 +3157,7 @@ Ajax.PeriodicalUpdater = Class.create(Ajax.Base, {
     var filter = Element.getStyle(element, 'filter');
     if (filter.length === 0) return 1.0;
     var match = (filter || '').match(/alpha\(opacity=(.*)\)/);
-    if (match[1]) return parseFloat(match[1]) / 100;
+    if (match && match[1]) return parseFloat(match[1]) / 100;
     return 1.0;
   }