Data.Heap.MaxPrioHeap can't type fmap

a.hs

import qualified Data.Heap as H

h :: H.MaxPrioHeap Int Char
h = H.fromList [(1, 'a')]

h' :: H.MaxPrioHeap Int String
h' = fmap (:[]) h

main :: IO ()
main = do
  print h
  print h'

result

a.hs:7:17:
    Couldn't match type `Char' with `[Char]'
    Expected type: H.HeapT (H.Prio H.FstMaxPolicy (Int, String)) Char
      Actual type: H.MaxPrioHeap Int Char
    In the second argument of `fmap', namely `h'
    In the expression: fmap (: []) h
    In an equation for h': h' = fmap (: []) h