diff --git a/src/calibre/srv/legacy.py b/src/calibre/srv/legacy.py
index 6c5aa803c9..72c6153e4d 100644
--- a/src/calibre/srv/legacy.py
+++ b/src/calibre/srv/legacy.py
@@ -254,9 +254,13 @@ def mobile(ctx, rd):
 @endpoint('/browse/{+rest=""}')
 def browse(ctx, rd, rest):
     if rest.startswith('book/'):
+        try:
+            book_id = int(rest[5:])
+        except Exception:
+            raise HTTPRedirect(ctx.url_for(None))
         # implementation of https://bugs.launchpad.net/calibre/+bug/1698411
         # redirect old server book URLs to new URLs
-        redirect = ctx.url_for(None) + '#book_id=' + int(rest[5:]) + "&panel=book_details"
+        redirect = ctx.url_for(None) + f'#book_id={book_id}&panel=book_details'
         from lxml import etree as ET
         return html(ctx, rd, endpoint,
                  E.html(E.head(