diff --git a/src/borg/helpers/fs.py b/src/borg/helpers/fs.py
index 18fe82407..d69a88715 100644
--- a/src/borg/helpers/fs.py
+++ b/src/borg/helpers/fs.py
@@ -230,7 +230,7 @@ def remember(self, *, id, info):
                      chunks / chunks_healthy list
                      hlid
         """
-        assert isinstance(id, self.id_type), f"key is {key!r}, not of type {self.key_type}"
+        assert isinstance(id, self.id_type), f"id is {id!r}, not of type {self.id_type}"
         assert isinstance(info, self.info_type), f"info is {info!r}, not of type {self.info_type}"
         self._map[id] = info
 
@@ -238,7 +238,7 @@ def retrieve(self, id, *, default=None):
         """
         retrieve stuff to use it in a (usually contentless) item.
         """
-        assert isinstance(id, self.id_type)
+        assert isinstance(id, self.id_type), f"id is {id!r}, not of type {self.id_type}"
         return self._map.get(id, default)