[Solved] Learn Responsive Web Design by Building a Piano - Step 6

UPDATE:
I figured it out, but I won’t delete this in case anyone else encounters the same thing, and in the name of lesson content feedback. To assign multiple classes, the syntax should be:
class="class1 class2"
rather than
class="class1" class="class2" —or— class="class1, class2"

To whom it may concern: this question is not phrased clearly; it asks us to assign multiple classes without ever having told us how. It’s a perfectly reasonable assumption that one would just provide two class indicators within the tag, as I tried to do. Please update the question itself (to include the formatting instruction) or at least make the error message more useful when users make this mistake. The existing error message was a total red herring.

------ORIGINAL POST------

Tell us what’s happening:

Am I crazy? My second .key HAS the .black–key class, right? Why can’t I move forward?

Your code so far

<!-- file: index.html -->
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <title>Piano</title>
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
  </head>
  <body>

<!-- User Editable Region -->

    <div id="piano">
      <div class="keys">
        <div class="key"></div>
        <div class="key" class="black--key"></div>
        <div class="key" class="black--key"></div>
        <div class="key"></div>
        <div class="key" class="black--key"></div>
        <div class="key" class="black--key"></div>
        <div class="key" class="black--key"></div>
      </div>
    </div>

<!-- User Editable Region -->

  </body>
</html>
/* file: styles.css */

Your browser information:

User Agent is: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/121.0.0.0 Safari/537.36

Challenge Information:

Learn Responsive Web Design by Building a Piano - Step 6