当我通过Nothing时，编译器抱怨无法推断出Maybe的类型

Question

我收到一个我不明白的错误。我正在使用的库具有以下功能：

getComputedStyle ::
             (MonadIO m, IsElement element, ToJSString pseudoElement) =>
               Window ->
                 Maybe element ->
                   Maybe pseudoElement -> m (Maybe CSSStyleDeclaration)

当我这样称呼时：

Window.getComputedStyle window (Just element) Nothing

我收到此错误：

Could not deduce (GHCJS.DOM.Types.ToJSString pseudoElement0)
  arising from a use of ‘getComputedStyle’
from the context (Control.Monad.IO.Class.MonadIO m,
                  IsElement element)
  bound by the inferred type of
           isBold :: (Control.Monad.IO.Class.MonadIO m, IsElement element) =>
                     Window -> element -> m (Maybe CSSStyleDeclaration)
  at src/Main.hs:(39,1)-(40,61)
The type variable ‘pseudoElement0’ is ambiguous
Note: there are several potential instances:
  instance GHCJS.DOM.Types.ToJSString
             ghcjs-base-0.2.0.0:Data.JSString.Internal.Type.JSString

    -- Defined in ‘GHCJS.DOM.Types’
  instance GHCJS.DOM.Types.ToJSString
      ghcjs-base-0.2.0.0:Data.JSString.Internal.Type.JSString
    -- Defined in ‘GHCJS.DOM.Types’
  instance GHCJS.DOM.Types.ToJSString
             text-1.2.1.1:Data.Text.Internal.Text
    -- Defined in ‘GHCJS.DOM.Types’
  instance GHCJS.DOM.Types.ToJSString [Char]

    -- Defined in ‘GHCJS.DOM.Types’
 ...

为什么编译器关心类型？我传递了一个Nothing值，因此Just值的类型并不真正相关。