GHC internal error while building darcsden

I get an internal error while building darcsden with GHC 8.0.1:

$ darcs get http://hub.darcs.net/simon/darcsden-ghc8 [--lazy]
$ cd darcsden-ghc8
$ stack --stack-yaml=stack-ghc8.yaml build
...
[12 of 56] Compiling DarcsDen.Backend.Transient ( src/DarcsDen/Backend/Transient.hs, .stack-work/dist/i386-linux/Cabal-1.24.0.0/build/DarcsDen/Backend/Transient.o )                                          
                                                                                                                                                                                                              
/home/simon/src/darcsden/src/DarcsDen/Backend/Transient.hs:10:33: error:                                                                                                                                      
    • GHC internal error: ‘BackendTransientM’ is not in scope during type checking, but it passed the renamer                                                                                                 
      tcl_env of environment: [ajE2 :-> Type variable ‘bt’ = bt]                                                                                                                                              
    • In the first argument of ‘MonadIO’, namely ‘BackendTransientM bt’                                                                                                                                       
      In the type ‘(BT bt, MonadIO (BackendTransientM bt))’                                                                                                                                                   
      In the type declaration for ‘BTIO’                                                                                                                                                                      
                                                                                                                                                                                                              
/home/simon/src/darcsden/src/DarcsDen/Backend/Transient.hs:44:1: error:                                                                                                                                       
    • Non type-variable argument                                                                                                                                                                              
        in the constraint: Functor (BackendTransientM bt)                                                                                                                                                     
      (Use FlexibleContexts to permit this)                                                                                                                                                                   
    • In the context: (Functor (BackendTransientM bt),                                                                                                                                                        
                       Monad (BackendTransientM bt))                                                                                                                                                          
      While checking the super-classes of class ‘BackendTransient’                                                                                                                                            
      In the class declaration for ‘BackendTransient’                                                                                                                                                         
                                                                                                                                                                                                              
--  While building package darcsden-1.1.99 using:                                                                                                                                                             
      /home/simon/.stack/setup-exe-cache/i386-linux/setup-Simple-Cabal-1.24.0.0-ghc-8.0.1 --builddir=.stack-work/dist/i386-linux/Cabal-1.24.0.0 build lib:darcsden exe:darcsden exe:darcsden-post-hook exe:da...
    Process exited with code: ExitFailure 1