diff --git a/capa/perf.py b/capa/perf.py
index 067c4f37..cb0e89ec 100644
--- a/capa/perf.py
+++ b/capa/perf.py
@@ -1,6 +1,7 @@
 import collections
 from typing import Dict
 
+# this structure is unstable and may change before the next major release.
 counters: Dict[str, int] = collections.Counter()