diff --git a/SC2021.md b/SC2021.md
index fd3ef06..1f2838b 100644
--- a/SC2021.md
+++ b/SC2021.md
@@ -21,4 +21,8 @@ Unless you want to operate on literal square brackets, don't include them.
 
 If you do want to replace literal square brackets, reorder the expression (e.g. `a-z[]` to make it clear that the brackets are not special).
 
-ShellCheck does not warn about correct usage of `[..]` in character and equivalence classes like `[:lower:]` and `[=e=]`.
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+ShellCheck does not warn about correct usage of `[..]` in character and equivalence classes like `[:lower:]` and `[=e=]`.
+
+### Note:
+
+Busybox requires `CONFIG_FEATURE_TR_CLASSES` for this to work. If busybox was compiled without this feature, character classes will obviously not be available, and the correct usage is to use `a-z`, not `[a-z]`.
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