Asynchronous sydTest hangs

Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: GHC.Internal.IO.mask3, called from Control.Concurrent.Async.Internal.catchAll, called from Control.Concurrent.Async.Internal.concurrently', called from Control.Concurrent.Async.Internal.concurrently, called from Test.Syd.Runner.Asynchronous.runSpecForestInterleavedWithOutputAsynchronously, called from GHC.Internal.Control.Exception.Base.bracket1, called from GHC.Internal.System.Environment.withArgs2, called from Test.Syd.Runner.withNullArgs, called from Test.Syd.Runner.sydTestOnce, called from Test.Syd.Runner.sydTestResult, called from Test.Syd.sydTestWith, called from Main.sydTest, called from Main.tt, called from Main.mainL, called from Main.main, called from GHC.Internal.TopHandler.runMainIO1