enhancing a simple php application => adding search controller view => escaping output review

Here's the quiz question: "A visitor to our site has just performed a search for chocolate, which has returned no results. Change this code to display the search term in the search box again, making sure to protect the page against malicious code that might have been entered."

I have no idea on what to do, I've looked at other forum topics and I'm almost positive my answer is right. Is there something that I'm missing?

<input type="text" name="s"  value="<?php if(isset($search_term)) { echo htmlspecialchars($search_term); } ?>">

I've also tried replacing the $search_term variable with $_GET and $s and they all say the same thing:

'To place text inside a text input field, you should give it a value attribute.'

Thanks.