diff --git a/SC2018.md b/SC2018.md
index 3e21992..a613a14 100644
--- a/SC2018.md
+++ b/SC2018.md
@@ -28,4 +28,6 @@ Instead, you can use `[:lower:]` and `[:upper:]` to explicitly specify case:
 
 If you don't want `a-z` to match `é` or `A-Z` to match `Ñ`, you can ignore this message.
 
+As of 2019-09-08, BusyBox `tr` does not support character classes, so you would have to ignore this message.
+
 Note that the examples used here are multibyte characters in UTF-8. Many implementations (including GNU) fails to deal with them. 
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