testing functions that return a Maybe Monad

Say I have a function:

safeHead :: [a] -> Maybe a
safeHead [] = Nothing
safeHead xs = Just $ head xs

And a test:

describe "Example.safeHead" $ do
  it "returns the head" $ do
    safeHead [1,2,3] `shouldBe` Just 1

  it "returns Nothing for an empty list" $
    safeHead [] `shouldBe` Nothing

This however produces:

No instance for (Eq a0) arising from a use of ‘shouldBe’
The type variable ‘a0’ is ambiguous
Note: there are several potential instances:
  instance (Eq a, Eq b) => Eq (Either a b)
    -- Defined in ‘Data.Either’
  instance forall (k :: BOX) (s :: k). Eq (Data.Proxy.Proxy s)
    -- Defined in ‘Data.Proxy’
  instance (GHC.Arr.Ix i, Eq e) => Eq (GHC.Arr.Array i e)
    -- Defined in ‘GHC.Arr’
  ...plus 88 others
In the second argument of ‘($)’, namely
  ‘safeHead [] `shouldBe` Nothing’
In a stmt of a 'do' block:
  it "returns Nothing for an empty list"
  $ safeHead [] `shouldBe` Nothing
In the second argument of ‘($)’, namely
  ‘do { it "returns the head"
        $ do { safeHead [...] `shouldBe` Just 1 };
        it "returns Nothing for an empty list"
        $ safeHead [] `shouldBe` Nothing }’

Why? And how can I fix it?