'prototype.replace() not working in Google Tab Manager

I have a very simple Custom JavaScript variable (which is a small function) with prototype.replace() that returns undefined in GTM:

function(){
  
  var stringElem = {{Click Element}};
    
  return stringElem.replace( 'HTMLSpanElement: ', '' );

}

No matter what I do, it returns undefined. {{Click Element}} is in dataLayer and is showing a non-empty string value. What am I doing wrong?

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