diff --git a/lib/Database/PostgreSQL/Opium.hs b/lib/Database/PostgreSQL/Opium.hs
index c77c514..790ca83 100644
--- a/lib/Database/PostgreSQL/Opium.hs
+++ b/lib/Database/PostgreSQL/Opium.hs
@@ -2,51 +2,71 @@
 {-# LANGUAGE DefaultSignatures #-}
 {-# LANGUAGE FlexibleContexts #-}
 {-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE KindSignatures #-}
+{-# LANGUAGE OverloadedStrings #-}
 {-# LANGUAGE ScopedTypeVariables #-}
 {-# LANGUAGE TypeOperators #-}
 
 module Database.PostgreSQL.Opium
-  ( FromField (..)
+  ( Error (..)
+  , FieldError (..)
+  , FromField (..)
   , FromRow (..)
   , fetch_
   )
   where
 
-import Control.Monad.Trans.Maybe (MaybeT (..))
+import Control.Monad.Trans.Except (ExceptT (..), except, runExceptT)
 import Data.ByteString (ByteString)
-import Data.Proxy (Proxy (Proxy))
+import Data.Proxy (Proxy (..))
+import Data.Text (Text)
 import Database.PostgreSQL.LibPQ
-  ( Connection
+  ( Column
+  , Connection
   , Result
   , Row
   )
 import GHC.Generics (C, D, Generic, K1 (..), M1 (..), Meta (..), Rec0, Rep, S, to, (:*:) (..))
-import GHC.TypeLits (KnownSymbol, symbolVal)
-import Text.Printf (printf)
+import GHC.TypeLits (KnownSymbol, Symbol, symbolVal)
 
 import qualified Data.Text as Text
 import qualified Data.Text.Encoding as Encoding
 import qualified Database.PostgreSQL.LibPQ as LibPQ
 
-import Database.PostgreSQL.Opium.FromField (FromField (..))
+import Database.PostgreSQL.Opium.Error (Error (..))
+import Database.PostgreSQL.Opium.FromField (FieldError (..), FromField (..))
 
-fetch_ :: FromRow a => Connection -> ByteString -> IO (Maybe [a])
-fetch_ conn query = runMaybeT $ do
-  result <- MaybeT $ LibPQ.execParams conn query [] LibPQ.Text
-  MaybeT $ fetchResult result
+execParams :: Connection -> ByteString -> IO (Either Error Result)
+execParams conn query = do
+  LibPQ.execParams conn query [] LibPQ.Text >>= \case
+    Nothing ->
+      pure $ Left ErrorNoResult
+    Just result -> do
+      status <- LibPQ.resultStatus result
+      mbMessage <- LibPQ.resultErrorMessage result
+      case mbMessage of
+        Just "" -> pure $ Right result
+        Just message -> pure $ Left $ ErrorInvalidResult status $ Encoding.decodeUtf8 message
+        Nothing -> pure $ Right result
 
-fetchResult :: FromRow a => Result -> IO (Maybe [a])
+fetch_ :: FromRow a => Connection -> ByteString -> IO (Either Error [a])
+fetch_ conn query = runExceptT $ do
+  result <- ExceptT $ execParams conn query
+  ExceptT $ fetchResult result
+
+fetchResult :: FromRow a => Result -> IO (Either Error [a])
 fetchResult result = do
   nRows <- LibPQ.ntuples result
-  runMaybeT $ mapM (MaybeT . flip fromRow result) [0..nRows - 1]
+  runExceptT $ mapM (ExceptT . flip fromRow result) [0..nRows - 1]
 
 class FromRow a where
-  fromRow :: Row -> Result -> IO (Maybe a)
-  default fromRow :: (Generic a, FromRow' (Rep a)) => Row -> Result -> IO (Maybe a)
+  fromRow :: Row -> Result -> IO (Either Error a)
+  default fromRow :: (Generic a, FromRow' (Rep a)) => Row -> Result -> IO (Either Error a)
   fromRow row result = fmap to <$> fromRow' row result
 
 class FromRow' f where
-  fromRow' :: Row -> Result -> IO (Maybe (f p))
+  fromRow' :: Row -> Result -> IO (Either Error (f p))
 
 instance FromRow' f => FromRow' (M1 D c f) where
   fromRow' row result = fmap M1 <$> fromRow' row result
@@ -60,20 +80,48 @@ instance (FromRow' f, FromRow' g) => FromRow' (f :*: g) where
     z <- fromRow' row result
     pure $ (:*:) <$> y <*> z
 
-instance (KnownSymbol nameSym, FromField t) => FromRow' (M1 S ('MetaSel ('Just nameSym) nu ns dl) (Rec0 t)) where
-  fromRow' row result = do
-    mbColumn <- LibPQ.fnumber result name
-    case mbColumn of
-      Nothing -> pure Nothing
-      Just column -> do
-        mbField <- LibPQ.getvalue result row column
-        ty <- LibPQ.ftype result column
-        case fromField ty . Encoding.decodeUtf8 =<< mbField of
-          Nothing -> do
-            format <- LibPQ.fformat result column
-            printf "field %s: %s (oid: %s, format: %s)\n" (show name) (show mbField) (show ty) (show format)
-            pure Nothing
-          Just value ->
-            pure $ Just $ M1 $ K1 value
+-- TODO: Can we clean this up?
+
+decodeField
+  :: FromField t
+  => Text
+  -> (Row -> Maybe t -> Either Error t')
+  -> Row
+  -> Result
+  -> IO (Either Error (M1 S ('MetaSel ('Just (nameSym :: Symbol)) nu ns dl) (Rec0 t') p))
+decodeField nameText g row result = runExceptT $ do
+    column <- getColumn
+    oid <- ExceptT $ Right <$> LibPQ.ftype result column
+    mbField <- getValue column
+    value <- case mbField of
+      Nothing ->
+        except $ g row Nothing 
+      Just field -> do
+        value <- except $ mapLeft (ErrorDecode row nameText) $ fromField oid $ Encoding.decodeUtf8 field
+        except $ g row $ Just value
+    pure $ M1 $ K1 value
     where
-      name = Encoding.encodeUtf8 $ Text.pack $ symbolVal (Proxy :: Proxy nameSym)
+      name = Encoding.encodeUtf8 nameText
+
+      getColumn :: ExceptT Error IO Column
+      getColumn = ExceptT $
+        maybe (Left $ ErrorMissingColumn nameText) Right <$> LibPQ.fnumber result name
+
+      getValue :: Column -> ExceptT Error IO (Maybe ByteString)
+      getValue column = ExceptT $ Right <$> LibPQ.getvalue result row column
+
+instance {-# OVERLAPPABLE #-} (FromField t, KnownSymbol nameSym) => FromRow' (M1 S ('MetaSel ('Just nameSym) nu ns dl) (Rec0 t)) where
+  fromRow' = decodeField nameText $ \row -> \case
+    Nothing -> Left $ ErrorUnexpectedNull row nameText
+    Just value -> Right value
+    where
+      nameText = Text.pack $ symbolVal (Proxy :: Proxy nameSym)
+
+instance {-# OVERLAPPING #-} (KnownSymbol nameSym, FromField t) => FromRow' (M1 S ('MetaSel ('Just nameSym) nu ns dl) (Rec0 (Maybe t))) where
+  fromRow' = decodeField nameText $ const Right
+    where
+      nameText = Text.pack $ symbolVal (Proxy :: Proxy nameSym)
+
+mapLeft :: (b -> c) -> Either b a -> Either c a
+mapLeft f (Left l) = Left $ f l
+mapLeft _ (Right r) = Right r
diff --git a/lib/Database/PostgreSQL/Opium/Error.hs b/lib/Database/PostgreSQL/Opium/Error.hs
new file mode 100644
index 0000000..e5c7f07
--- /dev/null
+++ b/lib/Database/PostgreSQL/Opium/Error.hs
@@ -0,0 +1,14 @@
+module Database.PostgreSQL.Opium.Error (Error (..)) where
+
+import Data.Text (Text)
+import Database.PostgreSQL.LibPQ (ExecStatus, Row)
+
+import Database.PostgreSQL.Opium.FromField (FieldError)
+
+data Error
+  = ErrorDecode Row Text FieldError
+  | ErrorNoResult
+  | ErrorInvalidResult ExecStatus Text
+  | ErrorMissingColumn Text
+  | ErrorUnexpectedNull Row Text
+  deriving (Eq, Show)
diff --git a/lib/Database/PostgreSQL/Opium/FromField.hs b/lib/Database/PostgreSQL/Opium/FromField.hs
index 9dd61e4..c06731c 100644
--- a/lib/Database/PostgreSQL/Opium/FromField.hs
+++ b/lib/Database/PostgreSQL/Opium/FromField.hs
@@ -1,13 +1,17 @@
-{-# LANGUAGE LambdaCase #-}
 {-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE UndecidableInstances #-}
 
-module Database.PostgreSQL.Opium.FromField (FromField (..)) where
+module Database.PostgreSQL.Opium.FromField
+  ( FieldError (..)
+  , FromField (..)
+  ) where
 
 import Data.Attoparsec.Text
   ( Parser
   , decimal
   , parseOnly
   , signed
+  , takeText
   )
 import Data.Text (Text)
 import Database.PostgreSQL.LibPQ (Oid)
@@ -18,23 +22,30 @@ import qualified Database.PostgreSQL.Opium.Oid as Oid
 (\/) :: (a -> Bool) -> (a -> Bool) -> a -> Bool
 p \/ q = \x -> p x || q x
 
-eitherToMaybe :: Either b a -> Maybe a
-eitherToMaybe = \case
-  Left _ -> Nothing
-  Right x -> Just x
+data FieldError
+  = FieldErrorUnexpectedNull
+  | FieldErrorInvalidOid Oid
+  | FieldErrorInvalidField Oid Text String
+  deriving (Eq, Show)
+
+mapLeft :: (b -> c) -> Either b a -> Either c a
+mapLeft f (Left l) = Left $ f l
+mapLeft _ (Right r) = Right r
 
 fromParser
   :: (Oid -> Bool)
   -> Parser a
   -> Oid
   -> Text
-  -> Maybe a
-fromParser validOid parser oid value
-  | validOid oid = eitherToMaybe $ parseOnly parser value
-  | otherwise = Nothing
+  -> Either FieldError a
+fromParser validOid parser oid field
+    | validOid oid =
+      mapLeft (FieldErrorInvalidField oid field) $ parseOnly parser field
+    | otherwise =
+      Left $ FieldErrorInvalidOid oid
 
 class FromField a where
-  fromField :: Oid -> Text -> Maybe a
+  fromField :: Oid -> Text -> Either FieldError a
 
 instance FromField Int where
   fromField = fromParser
@@ -42,11 +53,9 @@ instance FromField Int where
     (signed decimal)
 
 instance FromField Text where
-  fromField oid text =
-    if Oid.text oid || Oid.character oid || Oid.characterVarying oid then
-      Just text
-    else
-      Nothing
+  fromField = fromParser
+    (Oid.text \/ Oid.character \/ Oid.characterVarying)
+    takeText
 
 instance FromField String where
   fromField oid text = Text.unpack <$> fromField oid text
diff --git a/opium.cabal b/opium.cabal
index dda2388..4b97eb6 100644
--- a/opium.cabal
+++ b/opium.cabal
@@ -64,6 +64,7 @@ library
 
     -- Modules included in this library but not exported.
     other-modules:
+        Database.PostgreSQL.Opium.Error,
         Database.PostgreSQL.Opium.FromField,
         Database.PostgreSQL.Opium.Oid
 
diff --git a/test/Database/PostgreSQL/Opium/FromFieldSpec.hs b/test/Database/PostgreSQL/Opium/FromFieldSpec.hs
index 7a31547..c3d4d97 100644
--- a/test/Database/PostgreSQL/Opium/FromFieldSpec.hs
+++ b/test/Database/PostgreSQL/Opium/FromFieldSpec.hs
@@ -32,8 +32,8 @@ instance FromRow SingleString where
 
 shouldFetch :: (Eq a, FromRow a, Show a) => Connection -> ByteString -> [a] -> IO ()
 shouldFetch conn query expectedRows = do
-  Just actualRows <- Opium.fetch_ conn query
-  actualRows `shouldBe` expectedRows
+  actualRows <- Opium.fetch_ conn query
+  actualRows `shouldBe` Right expectedRows
 
 spec :: SpecWith Connection
 spec = do
diff --git a/test/Database/PostgreSQL/OpiumSpec.hs b/test/Database/PostgreSQL/OpiumSpec.hs
index 4f779b7..f09eeb3 100644
--- a/test/Database/PostgreSQL/OpiumSpec.hs
+++ b/test/Database/PostgreSQL/OpiumSpec.hs
@@ -7,7 +7,7 @@ module Database.PostgreSQL.OpiumSpec (spec) where
 import Data.Text (Text)
 import Database.PostgreSQL.LibPQ (Connection)
 import GHC.Generics (Generic)
-import Test.Hspec (SpecWith, describe, it, shouldBe)
+import Test.Hspec (SpecWith, describe, it, shouldBe, shouldSatisfy)
 
 import qualified Database.PostgreSQL.LibPQ as LibPQ
 import qualified Database.PostgreSQL.Opium as Opium
@@ -19,6 +19,16 @@ data Person = Person
 
 instance Opium.FromRow Person where
 
+newtype MaybeTest = MaybeTest
+  { a :: Maybe String
+  } deriving (Eq, Generic, Show)
+
+instance Opium.FromRow MaybeTest where
+
+isLeft :: Either a b -> Bool
+isLeft (Left _) = True
+isLeft _ = False
+
 spec :: SpecWith Connection
 spec = do
   describe "fromRow" $ do
@@ -26,12 +36,40 @@ spec = do
       Just result <- LibPQ.execParams conn "SELECT * FROM person" [] LibPQ.Text
 
       row0 <- Opium.fromRow @Person (LibPQ.Row 0) result
-      row0 `shouldBe` Just (Person "paul" 25)
+      row0 `shouldBe` Right (Person "paul" 25)
 
       row1 <- Opium.fromRow @Person (LibPQ.Row 1) result
-      row1 `shouldBe` Just (Person "albus" 103)
+      row1 `shouldBe` Right (Person "albus" 103)
+
+    it "decodes NULL into Nothing for Maybes" $ \conn -> do
+      Just result <- LibPQ.execParams conn "SELECT NULL AS a" [] LibPQ.Text
+
+      row <- Opium.fromRow (LibPQ.Row 0) result
+      row `shouldBe` Right (MaybeTest Nothing)
+
+    it "decodes values into Just for Maybes" $ \conn -> do
+      Just result <- LibPQ.execParams conn "SELECT 'abc' AS a" [] LibPQ.Text
+
+      row <- Opium.fromRow (LibPQ.Row 0) result
+      row `shouldBe` Right (MaybeTest $ Just "abc")
 
   describe "fetch_" $ do
     it "retrieves a list of rows" $ \conn -> do
       rows <- Opium.fetch_ conn "SELECT * FROM person"
-      rows `shouldBe` Just [Person "paul" 25, Person "albus" 103]
+      rows `shouldBe` Right [Person "paul" 25, Person "albus" 103]
+
+    it "fails for invalid queries" $ \conn -> do
+      rows <- Opium.fetch_ @Person conn "MRTLBRNFT"
+      rows `shouldSatisfy` isLeft
+
+    it "fails for missing columns" $ \conn -> do
+      rows <- Opium.fetch_ @Person conn "SELECT name FROM person"
+      rows `shouldBe` Left (Opium.ErrorMissingColumn "age")
+
+    it "fails for unexpected NULLs" $ \conn -> do
+      rows <- Opium.fetch_ @Person conn "SELECT NULL AS name, 0 AS age"
+      rows `shouldBe` Left (Opium.ErrorUnexpectedNull (LibPQ.Row 0) "name")
+
+    it "fails for the wrong column type" $ \conn -> do
+      rows <- Opium.fetch_ @Person conn "SELECT 'quby' AS name, 'indeterminate' AS age"
+      rows `shouldBe` Left (Opium.ErrorDecode (LibPQ.Row 0) "age" $ Opium.FieldErrorInvalidOid $ LibPQ.Oid 25)
diff --git a/test/SpecHook.hs b/test/SpecHook.hs
index d02b716..07fa76c 100644
--- a/test/SpecHook.hs
+++ b/test/SpecHook.hs
@@ -23,7 +23,7 @@ setupConnection = do
   conn <- LibPQ.connectdb $ Encoding.encodeUtf8 $ Text.pack dsn
   _ <- LibPQ.setClientEncoding conn "UTF8"
 
-  _ <- LibPQ.exec conn "CREATE TABLE person (name TEXT NOT NULL, age INT NOT NULL)"
+  _ <- LibPQ.exec conn "CREATE TABLE person (name TEXT NOT NULL, age INT NOT NULL, motto TEXT)"
   _ <- LibPQ.exec conn "INSERT INTO person VALUES ('paul', 25), ('albus', 103)"
 
   pure conn