SML - pattern matching is not exhaustive

Hvad er der galt med denne funktion:

fun ExtractOperands [] = []
  | ExtractOperands [H("",(_))] = []
  | ExtractOperands [H(Operands,[])] = []
  | ExtractOperands [H(Operands,(x :: xs))] =
    Operands :: (ExtractOperands [x]) @ (ExtractOperands xs);

Det er vidst noget med at jeg mangler at tjekke for noget, men hvad er det der er galt?