Blend tree: Compute Positions throwing warning, not working

Clicking on 'Compute Positions' inside a blend tree in mecanim (with any drop-down selection) does not alter motion positions as expected, and throws this warning:

Could not compute threshold for 'MovementTree' there is not enough data UnityEditor.GenericMenu:CatchMenu(Object, String[], Int32)

Same results with all 2D blend types. I've tried with baked XZ root motion both on and off. I've tried animation speeds of all 1, and of varying speeds.

Perhaps related, clicking 'Adjust Time Scale' -> 'Homogeneous Speed' gives this warning:

Could not adjust time scale for walk_high_backwards because it has no speed

I'm using Unity 5.4.0b15 Personal. Any help would be appreciated!

EDIT: Just to clarify, the warnings are issued for each blend tree in my scene, and each motion in the blend tree respectively.