getLast test clears until running getAt test - checked code from completed exercises, no help

[dfrho]

● GetAt › returns the node at given index

TypeError: Cannot set property 'next' of null

  79 |     let lastNode = this.getLast()
  80 |     let newNode = new Node(data)
> 81 |     lastNode.next = newNode
     |     ^
  82 |   }
  83 |
  84 |   getAt(index) {

  at LinkedList.insertLast (linkedlist/index.js:81:5)
  at Object.test (linkedlist/test.js:155:7)