Why do I get an Could not deduce (Ord a) error?

I am trying to find the list with the smallest sum of elements.:

shortest :: (Num a) => [[a]] -> [a]
shortest [] = []
shortest (x:xs) = if sum x < sum (shortest xs) then x else shortest xs

That gives me the following error:

Could not deduce (Ord a) arising from a use of `<'
from the context (Eq a)
  bound by the type signature for shortest :: Eq a => [[a]] -> [a]
  at code.hs:(8,1)-(9,71)
Possible fix:
  add (Ord a) to the context of
    the type signature for shortest :: Eq a => [[a]] -> [a]
In the expression: sum x < sum (shortest xs)
In the expression:
  if sum x < sum (shortest xs) then x else shortest xs
In an equation for `shortest':
    shortest (x : xs)
      = if sum x < sum (shortest xs) then x else shortest xs

Why doesn't the function typecheck?