Non-exhaustive patterns in function (Haskell)

I have two code snippets that throw the same error:

Prelude> sum' [] = 0
Prelude> sum' (x:xs) = x + sum' xs
Prelude> sum' [1,2,3,4,5]
*** Exception: <interactive>:142:1-25: Non-exhaustive patterns in function sum'

and the following as well:

Prelude> prod [] = 1
Prelude> prod (x:xs) = x * (prod xs)
Prelude> prod [1,2,3,4,5]
*** Exception: <interactive>:139:1-27: Non-exhaustive patterns in function prod

I must be missing a pattern, but what is it? Also, how do I such errors? How should I think when defining a function using pattern matching? (I'm asking for a methodolgy/technique)