diff --git a/ab-nginx.sh b/ab-nginx.sh
index d80f1a8..470dac6 100755
--- a/ab-nginx.sh
+++ b/ab-nginx.sh
@@ -92,7 +92,7 @@ checkExist 'file' './ab-nginx.params'
 
 # fix case of TLS13_ONLY var
 if [ "$TLS13_ONLY" ]; then
-    TLS13_ONLY=$( echo "$TLS13_ONLY" | tr "[:lower:]" "[:upper:]" )
+    TLS13_ONLY=$( printf "%s" "$TLS13_ONLY" | tr "[:lower:]" "[:upper:]" )
 fi
 
 # check for certs if using SSL