not an expression? Why?

Support.hs

data SupportData = SupportData
    { customer :: Entity Customer
    , followUps :: [Entity FollowUp]
    , customerFollowUps :: [(Entity FollowUp, Entity Customer)]
    }

getSupportRData :: CustomerId -> Handler SupportData
getSupportRData customerId = do
  customer_issues_followUps_list <- runDB $
    E.select $
    E.from $ \(c `E.InnerJoin` i `E.LeftOuterJoin` f) -> do
    E.on (i ^. IssueId ==. f ^. FollowUpIssueId)
    E.on (c ^. CustomerId ==. i ^. IssueCustomerId)
    E.where_ (i ^. IssueCustomerId ==. E.val customerId)
    return (i, f, c)
  let followUps = snd3 . unzip3 $ customer_issues_followUps_list
  customers_followUps <- forM followUps $ \followUp -> do
    customers <- runDB $
      E.select $
      E.from $ \c -> do
      E.where_ (c ^. CustomerId ==. E.val (followUpAuthor (entityVal followUp)))
      return $ (followUp, (fromMaybe (error "Foreign key violation: followUp -> customer") (listToMaybe customers)))
  let issues :: [Maybe (Entity Issue)] = map listToMaybe . group . sort . fst3 . unzip3 $ customer_issues_followUps_list
  let customer = listToMaybe . thd3 . unzip3 $ customer_issues_followUps_list
  return SupportData
    { customer = customer
    , followUps = followUps
    , customerFollowUps = customer_followUps
    }

Support_error

[18 of 19] Compiling Handler.Support  ( src/Handler/Support.hs, .stack-work/dist/x86_64-linux-tinfo6/Cabal-2.2.0.1/build/Handler/Support.o )

/home/hhefesto/dev/laurus-nobilis/src/Handler/Support.hs:18:5: error:
    The last statement in a 'do' block must be an expression
      customers <- runDB $ E.select $ E.from
                     $ \ c
                         -> do E.where_
                                 (c ^. CustomerId ==. E.val (followUpAuthor (entityVal followUp)))
                               return
                                 $ (followUp, 
                                    (fromMaybe
                                       (error "Foreign key violation: followUp -> customer")
                                       (listToMaybe customers)))
   |
18 |     customers <- runDB $
   |     ^^^^^^^^^^^^^^^^^^^^...