source subdirectory in package

Dear GHC developers,

I am testing Cabal in ghc-6.4. 
Consider a project of the modules        U1.hs, subdir/V1.hs
put to the source root of                $(HOME)/user1/
Provide some simplest source for U1, V1,
and the package description

-- u1.cabal --------------------- 
name:            u1
version:         0.0
build-depends:   data
exposed-modules: U1 V1
ghc-options:     -isubdir
---------------------------------

Then, 
  runhaskell Setup.hs configure --ghc --prefix=$HOME/t/user1/inst
and
  runhaskell Setup.hs build 
yield 

  Preprocessing library u1-0.0...
  can't find source for V1 in ["."]

Why this is so?

With kind regards,

-----------------
Serge Mechveliani
mechvel@xxxxxxxx