diff --git a/src/allmydata/util/fileutil.py b/src/allmydata/util/fileutil.py
index 26943212b..133d22710 100644
--- a/src/allmydata/util/fileutil.py
+++ b/src/allmydata/util/fileutil.py
@@ -615,6 +615,8 @@ else:
         precondition_abspath(replacement_path)
         precondition_abspath(backup_path)
 
+        if not os.path.exists(replacement_path):
+            raise ConflictError("Replacement file not found: %r" % (replacement_path,))
         try:
             os.rename(replaced_path, backup_path)
             rename_no_overwrite(replacement_path, replaced_path)