diff --git a/libatrade.cabal b/libatrade.cabal
index dbbca02..f37f4a0 100644
--- a/libatrade.cabal
+++ b/libatrade.cabal
@@ -15,6 +15,7 @@ cabal-version:       >=1.10
 
 library
   hs-source-dirs:      src
+  ghc-options:         -Wincomplete-patterns
   exposed-modules:     ATrade.Types
                      , ATrade.QuoteSource.Server
                      , ATrade.Broker.Protocol
@@ -37,7 +38,7 @@ library
 executable libatrade-exe
   hs-source-dirs:      app
   main-is:             Main.hs
-  ghc-options:         -threaded -rtsopts -with-rtsopts=-N
+  ghc-options:         -threaded -rtsopts -with-rtsopts=-N -Wincomplete-patterns
   build-depends:       base
                      , libatrade
                      , pretty-hex
@@ -69,7 +70,7 @@ test-suite libatrade-test
                      , bytestring
                      , monad-loops
                      , uuid
-  ghc-options:         -threaded -rtsopts -with-rtsopts=-N
+  ghc-options:         -threaded -rtsopts -with-rtsopts=-N -Wincomplete-patterns
   default-language:    Haskell2010
   other-modules:       ArbitraryInstances
                      , TestBrokerProtocol
diff --git a/src/ATrade/Broker/Protocol.hs b/src/ATrade/Broker/Protocol.hs
index c99334a..d3c226a 100644
--- a/src/ATrade/Broker/Protocol.hs
+++ b/src/ATrade/Broker/Protocol.hs
@@ -47,6 +47,7 @@ instance ToJSON BrokerServerRequest where
 data BrokerServerResponse = ResponseOrderSubmitted OrderId
   | ResponseOrderCancelled OrderId
   | ResponseNotifications [Notification]
+  | ResponseError T.Text
   deriving (Eq, Show)
 
 instance FromJSON BrokerServerResponse where
@@ -59,12 +60,16 @@ instance FromJSON BrokerServerResponse where
         return $ ResponseOrderCancelled oid
        | HM.member "notifications" obj -> do
         notifications <- obj .: "notifications"
-        ResponseNotifications <$> parseJSON notifications)
+        ResponseNotifications <$> parseJSON notifications
+       | HM.member "error" obj -> do
+        error <- obj .: "error"
+        ResponseError <$> parseJSON error)
 
 instance ToJSON BrokerServerResponse where
   toJSON (ResponseOrderSubmitted oid) = object [ "order-id" .= oid ]
   toJSON (ResponseOrderCancelled oid) = object [ "order-cancelled" .= oid ]
   toJSON (ResponseNotifications notifications) = object [ "notifications" .= notifications ]
+  toJSON (ResponseError errorMessage) = object [ "error" .= errorMessage ]
 
 data Notification = OrderNotification OrderId OrderState | TradeNotification Trade
   deriving (Eq, Show)
diff --git a/test/ArbitraryInstances.hs b/test/ArbitraryInstances.hs
index d8f7a02..7d3dafb 100644
--- a/test/ArbitraryInstances.hs
+++ b/test/ArbitraryInstances.hs
@@ -111,8 +111,9 @@ instance Arbitrary BrokerServerRequest where
 
 instance Arbitrary BrokerServerResponse where
   arbitrary = do
-    t <- choose (1, 3) :: Gen Int
+    t <- choose (1, 4) :: Gen Int
     if | t == 1 -> ResponseOrderSubmitted <$> arbitrary
        | t == 2 -> ResponseOrderCancelled <$> arbitrary
        | t == 3 -> ResponseNotifications <$> arbitrary
+       | t == 4 -> ResponseError <$> arbitrary