diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,8 @@
 
+## 1.0.1.0
+
+*   Update `IsMember` to show a nice custom type error.  Thanks @chshersh! [#5]
+
 ## 1.0.0.0
 
 *   Add the functions `relaxOpenUnion` and `relaxUnion` for being able to add
diff --git a/src/Data/WorldPeace.hs b/src/Data/WorldPeace.hs
--- a/src/Data/WorldPeace.hs
+++ b/src/Data/WorldPeace.hs
@@ -25,8 +25,8 @@
 each possible type in the 'OpenUnion':
 
 @
-  let strHandler = (\str -> \"got a String: \" '++' str) :: 'String' -> 'String'
-      intHandler = (\int -> \"got an Int: \" '++' 'show' int) :: 'Int' -> 'String'
+  let strHandler = (\\str -> \"got a String: \" '++' str) :: 'String' -> 'String'
+      intHandler = (\\int -> \"got an Int: \" '++' 'show' int) :: 'Int' -> 'String'
   in 'catchesOpenUnion' (strHandler, intHandler) u :: 'String'
 @
 
diff --git a/src/Data/WorldPeace/Union.hs b/src/Data/WorldPeace/Union.hs
--- a/src/Data/WorldPeace/Union.hs
+++ b/src/Data/WorldPeace/Union.hs
@@ -81,6 +81,7 @@
 import Data.Kind (Constraint)
 import Data.Proxy
 import Data.Typeable (Typeable)
+import GHC.TypeLits (ErrorMessage(..), TypeError)
 import Text.Read (Read(readPrec), ReadPrec, (<++))
 
 import Data.WorldPeace.Internal.Prism
@@ -131,6 +132,40 @@
   RIndex r (r ': rs) = 'Z
   RIndex r (s ': rs) = 'S (RIndex r rs)
 
+-- | Text of the error message.
+type NoElementError (r :: k) (rs :: [k]) =
+          'Text "You require open sum type to contain the following element:"
+    ':$$: 'Text "    " ':<>: 'ShowType r
+    ':$$: 'Text "However, given list can store elements only of the following types:"
+    ':$$: 'Text "    " ':<>: 'ShowType rs
+
+-- | This type family checks whether @a@ is inside @as@ and produces
+-- compile-time error if not.
+type family CheckElemIsMember (a :: k) (as :: [k]) :: Constraint where
+    CheckElemIsMember a as =
+      If (Elem a as) (() :: Constraint) (TypeError (NoElementError a as))
+
+-- | Type-level @if@.
+--
+-- >>> Refl :: If 'True String Double :~: String
+-- Refl
+-- >>> Refl :: If 'False String Double :~: Double
+-- Refl
+type family If (bool :: Bool) (thenCase :: k) (elseCase :: k) :: k where
+  If 'True thenCase _ = thenCase
+  If 'False _ elseCase = elseCase
+
+-- | Type-level version of the 'elem' function.
+--
+-- >>> Refl :: Elem String '[Double, String, Char] :~: 'True
+-- Refl
+-- >>> Refl :: Elem String '[Double, Char] :~: 'False
+-- Refl
+type family Elem (x :: k) (xs :: [k]) :: Bool where
+    Elem _ '[]       = 'False
+    Elem x (x ': xs) = 'True
+    Elem x (y ': xs) = Elem x xs
+
 -- | Change a list of types into a list of functions that take the given type
 -- and return @x@.
 --
@@ -151,7 +186,7 @@
 
 -- | This is a helpful 'Constraint' synonym to assert that @a@ is a member of
 -- @as@.  You can see how it is used in functions like 'openUnionLift'.
-type IsMember (a :: u) (as :: [u]) = UElem a as (RIndex a as)
+type IsMember (a :: u) (as :: [u]) = (CheckElemIsMember a as, UElem a as (RIndex a as))
 
 -- | A type family to assert that all of the types in a list are contained
 -- within another list.
@@ -435,7 +470,9 @@
 -- 'ElemRemove' gives you a way to specific types from a 'Union'.
 --
 -- Note that @'ElemRemove' a as@ doesn't force @a@ to be in @as@.  We are able
--- to try to pull out a 'Double' from a @'Union' 'Identity' \'['Double']@:
+-- to use 'unionRemove' to try to pull out a 'String' from a
+-- @'Union' 'Identity' \'['Double']@ (even though there is no way this 'Union'
+-- could contain a 'String'):
 --
 -- >>> let u = This (Identity 3.5) :: Union Identity '[Double]
 -- >>> unionRemove u :: Either (Union Identity '[Double]) (Identity String)
@@ -586,6 +623,8 @@
 ---------------
 
 -- | We can use @'Union' 'Identity'@ as a standard open sum type.
+--
+-- See the documentation for 'Union'.
 type OpenUnion = Union Identity
 
 -- | Case analysis for 'OpenUnion'.
@@ -645,11 +684,25 @@
 
 -- | Just like 'unionLift' but for 'OpenUnion'.
 --
+-- ==== __Examples__
+--
 -- Creating an 'OpenUnion':
 --
 -- >>> let string = "hello" :: String
 -- >>> openUnionLift string :: OpenUnion '[Double, String, Int]
 -- Identity "hello"
+--
+-- You will get a compile error if you try to create an 'OpenUnion' that
+-- doesn't contain the type:
+--
+-- >>> let float = 3.5 :: Float
+-- >>> openUnionLift float :: OpenUnion '[Double, Int]
+-- ...
+--     • You require open sum type to contain the following element:
+--           Float
+--       However, given list can store elements only of the following types:
+--           '[Double, Int]
+-- ...
 openUnionLift
   :: forall a as.
      IsMember a as
@@ -673,6 +726,18 @@
 -- >>> let p = openUnionLift double :: OpenUnion '[Double, String]
 -- >>> openUnionMatch p :: Maybe String
 -- Nothing
+--
+-- You will get a compile error if you try to pull out an element from
+-- the 'OpenUnion' that doesn't exist within it.
+--
+-- >>> let o2 = openUnionLift double :: OpenUnion '[Double, Char]
+-- >>> openUnionMatch o2 :: Maybe Float
+-- ...
+--     • You require open sum type to contain the following element:
+--           Float
+--       However, given list can store elements only of the following types:
+--           '[Double, Char]
+-- ...
 openUnionMatch
   :: forall a as.
      IsMember a as
diff --git a/test/Test/TypeErrors.hs b/test/Test/TypeErrors.hs
--- a/test/Test/TypeErrors.hs
+++ b/test/Test/TypeErrors.hs
@@ -26,18 +26,22 @@
         let u = This (Identity "hello") :: Union Identity '[String]
         shouldNotTypecheck
           (unionRemove u :: Either (Union Identity '[]) (Identity Double))
+
     , testCase "too few types in resulting union 2" $ do
         let u = This (Identity "hello") :: Union Identity '[String, Char, Double]
         shouldNotTypecheck
           (unionRemove u :: Either (Union Identity '[String]) (Identity Double))
+
     , testCase "too many types in resulting union 1" $ do
         let u = This (Identity "hello") :: Union Identity '[String]
         shouldNotTypecheck
           (unionRemove u :: Either (Union Identity '[String, String]) (Identity Double))
+
     , testCase "too many types in resulting union 2" $ do
         let u = This (Identity "hello") :: Union Identity '[String, Char, Double]
         shouldNotTypecheck
           (unionRemove u :: Either (Union Identity '[String, Char, Double]) (Identity Double))
+
     , testCase "does not pull out multiple" $ do
         let u = This (Identity "hello") :: Union Identity '[String, String, Double]
         shouldNotTypecheck
diff --git a/world-peace.cabal b/world-peace.cabal
--- a/world-peace.cabal
+++ b/world-peace.cabal
@@ -1,5 +1,5 @@
 name:                world-peace
-version:             1.0.0.0
+version:             1.0.1.0
 synopsis:            Open Union and Open Product Types
 description:         Please see <https://github.com/cdepillabout/world-peace#readme README.md>.
 homepage:            https://github.com/cdepillabout/world-peace