diff --git a/README.md b/README.md
index c4df9a1..7970a4f 100644
--- a/README.md
+++ b/README.md
@@ -31,6 +31,8 @@ Download <https://github.com/datajerk/c2d/archive/master.zip> and extract.
 
 ### Build Notes
 
+NOTE: cc65 2.13.3 required (no other version supported)
+
 Unix/Linux:
 
 	make