2 files changed, 12 insertions, 3 deletions

diff --git a/server/test/Thermoprint/Server/QueueSpec.hs b/server/test/Thermoprint/Server/QueueSpec.hs
index fd45e1b..c39083a 100644
--- a/server/test/Thermoprint/Server/QueueSpec.hs
+++ b/server/test/Thermoprint/Server/QueueSpec.hs
@@ -23,7 +23,7 @@ instance Arbitrary QueueEntry where
   arbitrary = QueueEntry <$> arbitrary <*> arbitrary
 
 instance Arbitrary PrintingError where
-  arbitrary = oneof [ return UnknownError
+  arbitrary = oneof [ return (IOError "dummy")
                     ]
 
 instance Arbitrary JobId where
diff --git a/spec/src/Thermoprint/API.hs b/spec/src/Thermoprint/API.hs
index 6c020fc..6f7fc02 100644
--- a/spec/src/Thermoprint/API.hs
+++ b/spec/src/Thermoprint/API.hs
@@ -1,5 +1,6 @@
 {-# LANGUAGE DeriveGeneric, DeriveAnyClass #-}
 {-# LANGUAGE TypeOperators, DataKinds #-}
+{-# LANGUAGE StandaloneDeriving #-}
 {-# LANGUAGE OverloadedStrings #-}
 
 -- | A specification of an API for interacting with a set of printers
@@ -59,8 +60,16 @@ data JobStatus = Queued PrinterId
                | Failed PrintingError
                deriving (Generic, Show, FromJSON, ToJSON)
 
-data PrintingError = UnknownError
-                   deriving (Typeable, Generic, NFData, Show, FromJSON, ToJSON, Exception)
+deriving instance Generic EncodingException
+deriving instance NFData EncodingException
+deriving instance FromJSON EncodingException
+deriving instance ToJSON EncodingException
+
+data PrintingError = IOError String -- ^ Not the actual error because we can't marshal that to JSON
+                   | EncError EncodingException -- ^ Could not encode some part of the 'Printout'
+                   deriving (Typeable, Generic, NFData, Show, FromJSON, ToJSON)
+
+instance Exception PrintingError
 
 type DraftTitle = Text