Steps to reproduce :

- import the project attached
- create a new animation controller
- create a new layer
- create a new avatar mask and uncheck the legs
- in the animation controller create a new state and assign a run animation
- make a transition from any state to the new state, preview it in the inspector and notice it plays fine
- in the previously created layer assign the avatar mask
- press play in the preview and notice the errors ( see the attached screenshot )

*Note :
Regression from 4.2RC4.