diff --git a/src/Language/GraphQL/Execute.hs b/src/Language/GraphQL/Execute.hs
index 6e46d7c..ac8f954 100644
--- a/src/Language/GraphQL/Execute.hs
+++ b/src/Language/GraphQL/Execute.hs
@@ -33,7 +33,7 @@ execute :: (MonadCatch m, VariableValue a, Serialize b)
     -> m (Either (ResponseEventStream m b) (Response b))
 execute schema' operationName subs document =
     case Transform.document schema' operationName subs document of
-        Left queryError -> pure $ singleError $ Transform.queryError queryError
+        Left queryError -> pure $ singleError $ show queryError
         Right transformed -> executeRequest transformed
 
 executeRequest :: (MonadCatch m, Serialize a)
diff --git a/src/Language/GraphQL/Execute/Internal.hs b/src/Language/GraphQL/Execute/Internal.hs
index 792a758..7e4e4a9 100644
--- a/src/Language/GraphQL/Execute/Internal.hs
+++ b/src/Language/GraphQL/Execute/Internal.hs
@@ -14,7 +14,7 @@ module Language.GraphQL.Execute.Internal
 import Control.Monad.Trans.State (modify)
 import Control.Monad.Catch (MonadCatch)
 import Data.Sequence ((|>))
-import Data.Text (Text)
+import qualified Data.Text as Text
 import Language.GraphQL.Execute.Coerce
 import Language.GraphQL.Error
     ( CollectErrsT
@@ -32,5 +32,6 @@ addError returnValue error' = modify appender >> pure returnValue
        { errors = errors |> error'
        }
 
-singleError :: Serialize b => forall a. Text -> Either a (Response b)
-singleError message = Right $ Response null $ pure $ Error message [] []
+singleError :: Serialize b => forall a. String -> Either a (Response b)
+singleError message =
+    Right $ Response null $ pure $ Error (Text.pack message) [] []
diff --git a/src/Language/GraphQL/Execute/Subscribe.hs b/src/Language/GraphQL/Execute/Subscribe.hs
index 44be965..0a2a681 100644
--- a/src/Language/GraphQL/Execute/Subscribe.hs
+++ b/src/Language/GraphQL/Execute/Subscribe.hs
@@ -15,8 +15,6 @@ import Data.HashMap.Strict (HashMap)
 import qualified Data.HashMap.Strict as HashMap
 import qualified Data.List.NonEmpty as NonEmpty
 import Data.Sequence (Seq(..))
-import Data.Text (Text)
-import qualified Data.Text as Text
 import Language.GraphQL.AST (Name)
 import Language.GraphQL.Execute.Coerce
 import Language.GraphQL.Execute.Execution
@@ -32,7 +30,7 @@ subscribe :: (MonadCatch m, Serialize a)
     => HashMap Name (Type m)
     -> Out.ObjectType m
     -> Seq (Transform.Selection m)
-    -> m (Either Text (ResponseEventStream m a))
+    -> m (Either String (ResponseEventStream m a))
 subscribe types' objectType fields = do
     sourceStream <- createSourceEventStream types' objectType fields
     traverse (mapSourceToResponseEvent types' objectType fields) sourceStream
@@ -51,7 +49,7 @@ createSourceEventStream :: MonadCatch m
     => HashMap Name (Type m)
     -> Out.ObjectType m
     -> Seq (Transform.Selection m)
-    -> m (Either Text (Out.SourceEventStream m))
+    -> m (Either String (Out.SourceEventStream m))
 createSourceEventStream _types subscriptionType@(Out.ObjectType _ _ _ fieldTypes) fields
     | [fieldGroup] <- OrderedMap.elems groupedFieldSet
     , Transform.Field _ fieldName arguments' _ <- NonEmpty.head fieldGroup
@@ -70,21 +68,19 @@ resolveFieldEventStream :: MonadCatch m
     => Type.Value
     -> Type.Subs
     -> Out.Subscribe m
-    -> m (Either Text (Out.SourceEventStream m))
+    -> m (Either String (Out.SourceEventStream m))
 resolveFieldEventStream result args resolver =
     catch (Right <$> runReaderT resolver context) handleEventStreamError
   where
     handleEventStreamError :: MonadCatch m
         => ResolverException
-        -> m (Either Text (Out.SourceEventStream m))
-    handleEventStreamError = pure . Left . Text.pack . displayException
+        -> m (Either String (Out.SourceEventStream m))
+    handleEventStreamError = pure . Left . displayException
     context = Type.Context
         { Type.arguments = Type.Arguments args
         , Type.values = result
         }
 
--- This is actually executeMutation, but we don't distinguish between queries
--- and mutations yet.
 executeSubscriptionEvent :: (MonadCatch m, Serialize a)
     => HashMap Name (Type m)
     -> Out.ObjectType m
diff --git a/src/Language/GraphQL/Execute/Transform.hs b/src/Language/GraphQL/Execute/Transform.hs
index ebbe633..62e5b54 100644
--- a/src/Language/GraphQL/Execute/Transform.hs
+++ b/src/Language/GraphQL/Execute/Transform.hs
@@ -25,7 +25,6 @@ module Language.GraphQL.Execute.Transform
     , QueryError(..)
     , Selection(..)
     , document
-    , queryError
     ) where
 
 import Control.Monad (foldM, unless)
@@ -71,8 +70,6 @@ data Selection m
     | SelectionField (Field m)
 
 -- | GraphQL has 3 operation types: queries, mutations and subscribtions.
---
--- Currently only queries and mutations are supported.
 data Operation m
     = Query (Maybe Text) (Seq (Selection m))
     | Mutation (Maybe Text) (Seq (Selection m))
@@ -98,10 +95,19 @@ data QueryError
     = OperationNotFound Text
     | OperationNameRequired
     | CoercionError
-    | TransformationError
     | EmptyDocument
     | UnsupportedRootOperation
 
+instance Show QueryError where
+    show (OperationNotFound operationName) = unwords
+        ["Operation", Text.unpack operationName, "couldn't be found in the document."]
+    show OperationNameRequired = "Missing operation name."
+    show CoercionError = "Coercion error."
+    show EmptyDocument =
+        "The document doesn't contain any executable operations."
+    show UnsupportedRootOperation =
+        "Root operation type couldn't be found in the schema."
+
 data Input
     = Int Int32
     | Float Double
@@ -114,17 +120,6 @@ data Input
     | Variable Type.Value
     deriving (Eq, Show)
 
-queryError :: QueryError -> Text
-queryError (OperationNotFound operationName) = Text.unwords
-    ["Operation", operationName, "couldn't be found in the document."]
-queryError OperationNameRequired = "Missing operation name."
-queryError CoercionError = "Coercion error."
-queryError TransformationError = "Schema transformation error."
-queryError EmptyDocument =
-    "The document doesn't contain any executable operations."
-queryError UnsupportedRootOperation =
-    "Root operation type couldn't be found in the schema."
-
 getOperation
     :: Maybe Full.Name
     -> NonEmpty OperationDefinition