To reproduce that use a direct link https://gchq.github.io/CyberChef/#recipe=HMAC(%7B'option':'','string':''%7D,'')&input=aGk
as you can see an uncaught TypeError look like
HMAC - TypeError in https://gchq.github.io/CyberChef/modules/Crypto.js on line 2.<br><br>Message: can't access property "blockSizeInBytes", n is undefined
the issue simple and persits because the Hashing Function is not validated is empty or not before using it in any operation
To reproduce that use a direct link https://gchq.github.io/CyberChef/#recipe=HMAC(%7B'option':'','string':''%7D,'')&input=aGk
as you can see an uncaught TypeError look like
the issue simple and persits because the Hashing Function is not validated is empty or not before using it in any operation