diff --git a/endpoints/compare/compare.php b/endpoints/compare/compare.php
index 5268795d..04a16a4e 100644
--- a/endpoints/compare/compare.php
+++ b/endpoints/compare/compare.php
@@ -106,7 +106,7 @@ class CompareBaseResponse extends TemplateResponse
     protected static function checkCompareString(string $val) : string
     {
         $val = urldecode($val);
-        if (preg_match('/[^\d\.:;]/', $val))
+        if (preg_match('/[^-?\d\.:;]/', $val))
             return '';
 
         return $val;