--prune-unsorted flag required on file with no annotations

[kozross]

I installed Liquid Haskell as per the instructions on Github using Stack. When I try to run stack exec liquid src/Test.hs, where Test.hs contains no annotations, I get the following error (many times):

:1:1-1:1: Error
  Constraint with free vars
  193
  [p, q]
  Try using the --prune-unsorted flag

Adding the --prune-unsorted flag makes this go away (and emit much more sensible suggestions). I'm a bit confused about this, and am unsure if it's a bug.