diff --git a/library/Icinga/Util/Format.php b/library/Icinga/Util/Format.php
index 756763028..46ccdadc4 100644
--- a/library/Icinga/Util/Format.php
+++ b/library/Icinga/Util/Format.php
@@ -29,6 +29,8 @@
 
 namespace Icinga\Util;
 
+use Icinga\Exception\ProgrammingError;
+
 class Format
 {
     const STANDARD_IEC = 0;
@@ -99,6 +101,9 @@ class Format
         if (! $timestamp) {
             return '-';
         }
+        if (! preg_match('~^\d+$~', $timestamp)) {
+            throw new ProgrammingError(sprintf('"%s" is not a number', $timestamp));
+        }
         $duration = time() - $timestamp;
         $prefix = '';
         if ($duration < 0) {