diff --git a/includes/theme.inc b/includes/theme.inc
index 35854d4a8e7bb88b4825562b3b60eed9d3e3dbae..96222de716b32a32ae97bfc5ce7cc439eb35a4d1 100644
--- a/includes/theme.inc
+++ b/includes/theme.inc
@@ -870,6 +870,7 @@ function theme($hook, $variables = array()) {
   }
 
   // Generate the output using either a function or a template.
+  $output = '';
   if (isset($info['function'])) {
     if (function_exists($info['function'])) {
       $output = $info['function']($variables);