Why doesn't my implementation of replace satisfy the test question?

I'm not sure why, I checked my syntax (though I could have a blind spot) but I'm not replacing an array nor am I reading from a file, so none of that code would apply to this example. This is just an implementation of the replace method, right?

var utilities = require("./utilities");

var mailValues = {};

mailValues.first_name = "Janet";

var emailTemplate = "Hi %first_name%! Thanks for completing this code challenge :)";

var mergedContent = utilities.merge(emailTemplate, mailValues);

//mergedContent === "Hi Janet! Thanks for completing this code challenge :)";

function merge(content, values) {

  content = content.replace("%first_name%", values.first_name);

  return content;
}


module.exports.merge = merge;

I thought maybe because I hardcoded "first_name" in utilities, it wasn't great, I'd have to edit it for any new parameter.

so I did this...

function merge(content, values) {

    for( var param in values) {
        content = content.replace("%"+param+"%", values[param]);
      return content;
  }
}

  module.exports.merge = merge;

That way, if there were any placeholders in the string, along as there's a value for it in the mailValues object, it would be swapped out with the actual value.

It also passes in my local environment, but doesn't satisfy the test.

I'm at a loss.