diff --git a/src/allmydata/frontends/magic_folder.py b/src/allmydata/frontends/magic_folder.py
index 4e0143b32..35db9597a 100644
--- a/src/allmydata/frontends/magic_folder.py
+++ b/src/allmydata/frontends/magic_folder.py
@@ -300,6 +300,7 @@ class Uploader(QueueMixin):
         if relpath_u is None:
             return
         precondition(isinstance(relpath_u, unicode), relpath_u)
+        precondition(not relpath_u.endswith(u'/'), relpath_u)
 
         d = defer.succeed(None)