diff --git a/qr_codes.module b/qr_codes.module
index b1c7244..2badcdc 100644
--- a/qr_codes.module
+++ b/qr_codes.module
@@ -138,8 +138,8 @@ function qr_codes_generate($data, $width, $height, $margin = 0) {
  */
 function theme_qr_codes($variables) {
   $data = $variables['data'];
-  $width = $variables['width'];
-  $height = $variables['height'];
+  $width = isset($variables['width']) ? $variables['width'] : 150;
+  $height = isset($variables['height']) ? $variables['height'] : 150;
   $margin = $variables['margin'];
   $alt = isset($variables['alt']) ? $variables['alt'] : '';
   $title = isset($variables['title']) ? $variables['title'] : '';