Couldn't match type ‘r’ with ‘Extend "pitch" Pitch (r :- "pitch")

[meditans]

Hi, the title is the error message I get when I try to write a function like:

c :: forall r. Forall (r :- "pitch") Default => Rec r
c = pitch := C .| (def :: Rec (r :- "pitch"))

The complete error message is:

src/LibCTRex.hs:54:5: Couldn't match type ‘r’ with ‘Extend "pitch" Pitch (r :- "pitch")’ …
      ‘r’ is a rigid type variable bound by
          the type signature for c :: Forall (r :- "pitch") Default => Rec r
          at /home/carlo/code/haskell/prova/musicRecord/src/LibCTRex.hs:53:13
    Expected _type:_ Rec r
      Actual type: Rec (("pitch" '::= Pitch) :| (r :- "pitch"))
    Relevant bindings include
      c :: Rec r
        (bound at /home/carlo/code/haskell/prova/musicRecord/src/LibCTRex.hs:54:1)
    In the expression: pitch := C .| (def :: Rec (r :- "pitch"))
    In an equation for ‘c’:
        c = pitch := C .| (def :: Rec (r :- "pitch"))
Compilation failed.

Now, shouldn't there be an isomorphism, in general, between those types? How
could I write one on my own?