diff --git a/personal_digest.module b/personal_digest.module
index 48ca350..ce935c5 100644
--- a/personal_digest.module
+++ b/personal_digest.module
@@ -205,7 +205,7 @@ function personal_digest_mail($key, &$message, $params) {
     if (empty($params['last'])) {
       // Estimate the time to send from if this is the first mail.
       $str = $params['weeks_interval'] . " weeks";
-      $params['last'] = strtotime(-$str);
+      $params['last'] = strtotime("-" . $str);
     }
 
     $message['subject'] = t(