strange non-exhaustive error with GHC 7

a.hs

addOrAssignAmountAndCheckAssertionB :: Posting -> Balancing s Posting
addOrAssignAmountAndCheckAssertionB p@Posting{paccount=acc, pamount=amt, pbalanceassertion=mba}  -- line 730
  | hasAmount p = do
      newbal <- addAmountB acc amt
      whenM (R.reader bsAssrt) $ checkBalanceAssertionB p newbal
      return p
  | Nothing <- mba = return p
  | Just BalanceAssertion{baamount,batotal} <- mba = do
      (diff,newbal) <- case batotal of
        True  -> do
          -- a total balance assignment
          let newbal = Mixed [baamount]
          diff <- setAmountB acc newbal
          return (diff,newbal)
        False -> do
          -- a partial balance assignment
          oldbalothercommodities <- filterMixedAmount ((acommodity baamount /=) . acommodity) <$> getAmountB acc
          let assignedbalthiscommodity = Mixed [baamount]
              newbal = oldbalothercommodities + assignedbalthiscommodity
          diff <- setAmountB acc newbal
          return (diff,newbal)
      let p' = p{pamount=diff, poriginal=Just $ originalPosting p}
      whenM (R.reader bsAssrt) $ checkBalanceAssertionB p' newbal
      return p'

{-
GHC 7.10.3:
    /home/simon/src/hledger/hledger-lib/Hledger/Data/Journal.hs:730:1: Warning:
        Pattern match(es) are non-exhaustive
        In an equation for ‘addOrAssignAmountAndCheckAssertionB’:
            Patterns not matched: Posting _ _ _ _ _ _ _ _ _ _ _

GHC 8+: ok
-}