diff --git a/lib/SimpleSAML/SessionHandlerCookie.php b/lib/SimpleSAML/SessionHandlerCookie.php
index 3cef005df778c43a81c4f34c81e18ce20cc27bd9..76cbecbec479b2fb69363a431c0a8a0f2729c7b3 100644
--- a/lib/SimpleSAML/SessionHandlerCookie.php
+++ b/lib/SimpleSAML/SessionHandlerCookie.php
@@ -77,7 +77,7 @@ abstract class SessionHandlerCookie extends SessionHandler
             }
 
             // check if we have a valid session id
-            if (!self::isValidSessionID($this->session_id)) {
+            if (!is_null($this->session_id) && !self::isValidSessionID($this->session_id)) {
                 // invalid, disregard this session
                 return null;
             }