Sus4 Chord toString() throws an error

[Eerovil]

musictheory.js:1770 Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'copy')
    at Scale.degree (musictheory.js:1770:1)
    at Chord.generateNotes (musictheory.js:2743:1)
    at get notes [as notes] (musictheory.js:2721:1)
    at Chord.getNoteNames (musictheory.js:2760:1)
    at Chord.toString (musictheory.js:3207:1)
    at makeChords (chords.ts:885:40)
    at chords.ts:973:18
    at Generator.next (<anonymous>)
    at wrap-event-listener-function.js:1:1
    at new Promise (<anonymous>)
    ```