diff --git a/c3stream.cs b/c3stream.cs
index 22670ee..88e7f77 100644
--- a/c3stream.cs
+++ b/c3stream.cs
@@ -82,7 +82,7 @@ namespace c3stream {
 
 		public static void UpdateCookie(HttpRequest request, HttpResponse response, string redirectUri) {
 			//if new bookmark is in uri
-			if (request.Query.ContainsKey("bookmark")) {
+			if (request.Query.ContainsKey("bookmark") && Guid.TryParseExact(request.Query["bookmark"], "D", out _)) {
 				response.Cookies.Append("bookmark", request.Query["bookmark"], new CookieOptions {Expires = DateTimeOffset.MaxValue});
 				response.Redirect(redirectUri);
 			}