diff --git a/letsencrypt.sh b/letsencrypt.sh
index 751c5e3..07d8b1d 100755
--- a/letsencrypt.sh
+++ b/letsencrypt.sh
@@ -129,7 +129,7 @@ _exiterr() {
 # Encode data as url-safe formatted base64
 urlbase64() {
   # urlbase64: base64 encoded string with '+' replaced with '-' and '/' replaced with '_'
-  openssl base64 -e | tr -d '\n\r' | sed 's/=*$//g' | tr '+/' '-_'
+  openssl base64 -e | tr -d '\n\r' | sed -e 's:=*$::g' -e 'y:+/:-_:'
 }
 
 # Convert hex string to binary data