diff --git a/app/scodoc/html_sidebar.py b/app/scodoc/html_sidebar.py
index c0c732ce..b64cefc6 100755
--- a/app/scodoc/html_sidebar.py
+++ b/app/scodoc/html_sidebar.py
@@ -88,8 +88,10 @@ def retreive_formsemestre_from_request() -> int:
 
     if formsemestre_id is None:
         return None  # no current formsemestre
-
-    return int(formsemestre_id)
+    try:
+        return int(formsemestre_id)
+    except ValueError:
+        return None  # no current formsemestre
 
 
 def sidebar_common():