diff --git a/libcomain b/libcomain
index 1c461c5..2faac22 100644
--- a/libcomain
+++ b/libcomain
@@ -33,12 +33,12 @@ warn()
 
 upper()
 {
-    printf "%s" "$@" | tr [:lower:] [:upper:]
+    printf "%s" "$@" | tr '[:lower:]' '[:upper:]'
 }
 
 sanitize()
 {
-    printf "%s" "$@" | tr [:lower:] [:upper:] | tr -c [:alnum:] _
+    printf "%s" "$@" | tr '[:lower:]' '[:upper:]' | tr -c '[:alnum:]' _
 }
 
 quote()