diff --git a/README.md b/README.md
index 3cb4d23..7ed685c 100644
--- a/README.md
+++ b/README.md
@@ -791,7 +791,7 @@ head -c 32 < /dev/urandom | xxd -p -c 32
 or make it alpha-numeric
 
 ```sh
-head -c 32 < /dev/urandom | base64 | tr -dc '[:alpha:]' | head -c 16
+head -c 32 < /dev/urandom | base64 | tr -dc '[:alnum:]' | head -c 16
 ```
 
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