diff --git a/README.md b/README.md
--- a/README.md
+++ b/README.md
@@ -18,7 +18,7 @@
 ```haskell
 {-# LANGUAGE DeriveGeneric, OverloadedStrings #-}
 
-import Data.Aeson (ToJSON)
+import Data.Aeson (ToJSON, (.=))
 import Database.PostgreSQL.Simple
  -- For our automatic JSON instance:
 import GHC.Generics (Generic)
@@ -182,23 +182,27 @@
 three :: Ctx -> Snap ()
 three ctx =
    inTransaction ctx $ \conn -> do
-      [Only d] <- query_ conn " SELECT random () "
-      pure $ if d >= (0.5 :: Double)
-         then rspGood d
-         else rspBad $ DefaultApiErr_Custom internalServerError500 $
-            "number too small: "++ show d
+      [Only n] <- query_ conn " SELECT random () "
+      pure $ if n >= (0.5 :: Double)
+         then rspGood n
+         else rspBad $
+            DefaultApiErr_Custom
+               internalServerError500
+               "'n' too small"
+               [("n" .= n)]
 ```
 
     $ curl -v 'localhost:8000/three'
     [...]
-    < HTTP/1.1 500 Internal Server Error
+    < HTTP/1.1 200 OK
     [...]
-    {"errorCode":6,"errorVals":[],"errorMessage":"number too small: 0.163489528931677"}
+    {"result":0.594665706157684}
+
     $ curl -v 'localhost:8000/three'
     [...]
-    < HTTP/1.1 200 OK
+    < HTTP/1.1 500 Internal Server Error
     [...]
-    {"result":0.594665706157684}
+    {"errorCode":6,"errorVals":[["n",0.250084751285613]],"errorMessage":"'n' too small"}
 
 If you'd like to write your own `ApiErr` instance (which you probably should if
 you're building something "real":
diff --git a/gingersnap.cabal b/gingersnap.cabal
--- a/gingersnap.cabal
+++ b/gingersnap.cabal
@@ -1,5 +1,5 @@
 name:                gingersnap
-version:             0.2.0.1
+version:             0.2.1.0
 synopsis:            snap-core + aeson + postgresql-simple = delicious
 description:
   JSON API idioms for snap-core, preventing DB connection leaks.
diff --git a/src/Gingersnap/Core.hs b/src/Gingersnap/Core.hs
--- a/src/Gingersnap/Core.hs
+++ b/src/Gingersnap/Core.hs
@@ -149,7 +149,7 @@
    | DefaultApiErr_RequestNotJSONObject
    | DefaultApiErr_MalformedRequestValue Text JSON.Value
    | DefaultApiErr_UnexpectedError Text
-   | DefaultApiErr_Custom HTTP.Status String
+   | DefaultApiErr_Custom HTTP.Status String [(Text, JSON.Value)]
  deriving (Show, Eq)
 
 instance ApiErr DefaultApiErr where
@@ -193,8 +193,8 @@
           (5, HTTP.unprocessableEntity422, "Malformed value: "++show k, [
                "key" .= k, "value" .= v
              ])
-       DefaultApiErr_Custom s t ->
-          (6, s, t, [])
+       DefaultApiErr_Custom s t vals ->
+          (6, s, t, vals)
 
 -- | How we construct responses. You probably don't want to be constructing or
 --   inspecting them by hand; instead you can use 'rspGood', 'rspBadRollback', etc.