How do I return Nothing from a function?

When there is no edge in the graph this code fails.
*** Exception: Graph.hs:65:9-26: Irrefutable pattern failed for pattern (Data.Maybe.Just w)


*Main> :t weight
weight
  :: (Ix t1, Ix t2) => t1 -> t2 -> Array (t1, t2) (Maybe t) -> t

weight x y g = w
    where
        (Just w) = g!(x,y)

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