diff --git a/Lib/functools.py b/Lib/functools.py
index 1062a45..03de69a 100644
--- a/Lib/functools.py
+++ b/Lib/functools.py
@@ -141,7 +141,7 @@
 
         hits = misses = 0
         kwd_mark = object()             # separates positional and keyword args
-        lock = Lock()
+        lock = Lock()                   # needed because ordereddicts aren't threadsafe
 
         if maxsize is None:
             cache = dict()              # simple cache without ordering or size limit