diff --git a/python/lsst/daf/butler/datastores/file_datastore/retrieve_artifacts.py b/python/lsst/daf/butler/datastores/file_datastore/retrieve_artifacts.py
index 583d4ede16..fd76d0ad45 100644
--- a/python/lsst/daf/butler/datastores/file_datastore/retrieve_artifacts.py
+++ b/python/lsst/daf/butler/datastores/file_datastore/retrieve_artifacts.py
@@ -365,7 +365,8 @@ def calc_relative_paths(
         for p in paths:
             # It is an error if there is no relative path.
             rel = p.relative_to(root)
-            assert rel is not None
+            if rel is None:
+                raise RuntimeError(f"Unexepectedly unable to calculate relative path of {p} to {root}.")
             file_to_relative[p] = rel
         return file_to_relative