Spine setSkelton Error

I am trying to use Spine in the way I use sprites with setTexture with spritesheets. I am reassigning a Spine animation to one in a different file.

  this.load.setPath('assets/spine/pussycat/');
  var pussyCat = this.load.spine('pussycat', 'pussycat.json', ['pussycat.atlas'], true)
  this.load.setPath('assets/spine/owl/');
  this.load.spine('owl', 'owl-pro.json', ['owl-pro.atlas'], true)

Why does pussyCat.setSkeleton('owl', 'owl-pro.json').setAnimation(0, 'idle', true) work, but pussyCat.setSkeleton('owl', 'owl-pro.json', 'idle', true) result in an error?

Uncaught TypeError: Cannot set property 'rotation' of nul at SpineGameObjectWebGLRenderer [as renderWebGL] (<anonymous>:32282:27)

https://photonstorm.github.io/phaser3-docs/SpineGameObject.html#setSkeleton__anchor