Validation error; previewPlan does not allow "free"

Hi,

Setting previewPlan: free on render.yaml on a web service results in:

services[0].previewPlan free plan is not valid as a previewPlan

This seems to be a validation error because setting:

  1. plan: starter + previewPlan: free results in the error above.
  2. plan: free and not setting previewPlan works and the preview is on the free plan.

thx