Run-time error "window function first_value requires an OVER clause"

This is my second attempt at writing code to select the latest version of each of a given type of document. Both have resulted in run-time errors. Here is the current offending version of the code, and below is the run-time error being produced.

For reference, the schema includes the following columns, which are used in the snippet below:

```hs
, type' :: Columnar f AgreementType
, version :: Columnar f Natural
```

```hs
-- | The latest version of each of the requested agreement types.
latestVersions ::
[AgreementType] ->
Q Postgres UserAccountServiceDb s (AgreementT (QExpr Postgres s))
latestVersions types = do
(expectedType, expectedVersion) <- aggregate_
(\Schema.Agreement.Agreement{..} -> (group_ type', firstValue_ version))
$ orderBy_ (desc_ . (.version))
$ filter_ (\Schema.Agreement.Agreement{..} -> type' `in_` (val_ <$> types))
$ all_ userAccountServiceDb.agreementTable
filter_ (\Schema.Agreement.Agreement{..} ->
type' ==. expectedType &&. version ==. expectedVersion)
$ all_ userAccountServiceDb.agreementTable
```

```hs
SqlError {sqlState = "42809", sqlExecStatus = FatalError, sqlErrorMsg = "window function first_value requires an OVER clause", sqlErrorDetail = "", sqlErrorHint = ""}
```