diff --git a/jvm.cabal b/jvm.cabal
--- a/jvm.cabal
+++ b/jvm.cabal
@@ -1,5 +1,5 @@
 name:                jvm
-version:             0.2.0
+version:             0.2.1
 synopsis:            Call JVM methods from Haskell.
 description:         Please see README.md.
 homepage:            http://github.com/tweag/inline-java/tree/master/jvm#readme
@@ -25,6 +25,7 @@
   build-depends:
     base >= 4.7 && < 5,
     bytestring >=0.10,
+    choice >= 0.1,
     distributed-closure >=0.3,
     jni >= 0.3.0,
     singletons >= 2.0,
diff --git a/src/Language/Java.hs b/src/Language/Java.hs
--- a/src/Language/Java.hs
+++ b/src/Language/Java.hs
@@ -55,6 +55,7 @@
   , call
   , callStatic
   , jvalue
+  , CoercionFailure(..)
   , Coercible(..)
   , Reify(..)
   , Reflect(..)
@@ -66,10 +67,13 @@
 
 import Control.Distributed.Closure
 import Control.Distributed.Closure.TH
+import Control.Exception (Exception, throw)
 import Control.Monad
 import Data.Char (chr, ord)
+import qualified Data.Choice as Choice
 import qualified Data.Coerce as Coerce
 import Data.Int
+import Data.Typeable (Typeable, TypeRep, typeOf)
 import Data.Word
 import Data.ByteString (ByteString)
 import qualified Data.ByteString.Char8 as BS
@@ -78,15 +82,16 @@
 import Data.String (fromString)
 import qualified Data.Text.Foreign as Text
 import Data.Text (Text)
-#if ! __GLASGOW_HASKELL__ == 800
+#if ! (__GLASGOW_HASKELL__ == 800 && __GLASGOW_HASKELL_PATCHLEVEL1__ == 1)
 import qualified Data.Vector.Storable as Vector
 import Data.Vector.Storable (Vector)
 import qualified Data.Vector.Storable.Mutable as MVector
 import Data.Vector.Storable.Mutable (IOVector)
-import Foreign (FunPtr, Ptr, Storable, newForeignPtr, withForeignPtr)
+import Foreign (Ptr, Storable, withForeignPtr)
+import Foreign.Concurrent (newForeignPtr)
 #endif
 import Foreign.C (CChar)
-import Foreign.JNI
+import Foreign.JNI hiding (throw)
 import Foreign.JNI.Types
 import qualified Foreign.JNI.String as JNI
 import GHC.TypeLits (KnownSymbol, Symbol)
@@ -123,45 +128,64 @@
 -- | The identity instance.
 instance SingI ty => Coercible (J ty) ty
 
+-- | A JNI call may cause a (Java) exception to be raised. This module raises it
+-- as a Haskell exception wrapping the Java exception.
+data CoercionFailure = CoercionFailure
+  { coercionActual :: JValue
+  , coercionExpected :: TypeRep
+  }
+
+instance Exception CoercionFailure
+
+instance Show CoercionFailure where
+  show (CoercionFailure actual expected) =
+    "Can't coerce " ++ show actual ++ " to " ++ show expected ++ "."
+
+withTypeRep :: Typeable a => (TypeRep -> a) -> a
+withTypeRep f = let x = f (typeOf x) in x
+
 instance Coercible Bool ('Prim "boolean") where
   coerce x = JBoolean (fromIntegral (fromEnum x))
   unsafeUncoerce (JBoolean x) = toEnum (fromIntegral x)
-  unsafeUncoerce _ = error "unsafeUncoerce: value doesn't match target type."
+  unsafeUncoerce val = withTypeRep (throw . CoercionFailure val)
 instance Coercible CChar ('Prim "byte") where
   coerce = JByte
   unsafeUncoerce (JByte x) = x
-  unsafeUncoerce _ = error "unsafeUncoerce: value doesn't match target type."
+  unsafeUncoerce val = withTypeRep (throw . CoercionFailure val)
 instance Coercible Char ('Prim "char") where
   coerce x = JChar (fromIntegral (ord x))
   unsafeUncoerce (JChar x) = chr (fromIntegral x)
-  unsafeUncoerce _ = error "unsafeUncoerce: value doesn't match target type."
+  unsafeUncoerce val = withTypeRep (throw . CoercionFailure val)
 instance Coercible Word16 ('Prim "char") where
   coerce = JChar
   unsafeUncoerce (JChar x) = x
-  unsafeUncoerce _ = error "unsafeUncoerce: value doesn't match target type."
+  unsafeUncoerce val = withTypeRep (throw . CoercionFailure val)
 instance Coercible Int16 ('Prim "short") where
   coerce = JShort
   unsafeUncoerce (JShort x) = x
-  unsafeUncoerce _ = error "unsafeUncoerce: value doesn't match target type."
+  unsafeUncoerce val = withTypeRep (throw . CoercionFailure val)
 instance Coercible Int32 ('Prim "int") where
   coerce = JInt
   unsafeUncoerce (JInt x) = x
-  unsafeUncoerce _ = error "unsafeUncoerce: value doesn't match target type."
+  unsafeUncoerce val = withTypeRep (throw . CoercionFailure val)
 instance Coercible Int64 ('Prim "long") where
   coerce = JLong
   unsafeUncoerce (JLong x) = x
-  unsafeUncoerce _ = error "unsafeUncoerce: value doesn't match target type."
+  unsafeUncoerce val = withTypeRep (throw . CoercionFailure val)
 instance Coercible Float ('Prim "float") where
   coerce = JFloat
   unsafeUncoerce (JFloat x) = x
-  unsafeUncoerce _ = error "unsafeUncoerce: value doesn't match target type."
+  unsafeUncoerce val = withTypeRep (throw . CoercionFailure val)
 instance Coercible Double ('Prim "double") where
   coerce = JDouble
   unsafeUncoerce (JDouble x) = x
-  unsafeUncoerce _ = error "unsafeUncoerce: value doesn't match target type."
+  unsafeUncoerce val = withTypeRep (throw . CoercionFailure val)
 instance Coercible () 'Void where
   coerce = error "Void value undefined."
   unsafeUncoerce _ = ()
+instance Coercible (Choice.Choice a) ('Prim "boolean") where
+  coerce = coerce . Choice.toBool
+  unsafeUncoerce = Choice.fromBool . unsafeUncoerce
 
 -- | Get the Java class of an object or anything 'Coercible' to one.
 classOf
@@ -317,11 +341,7 @@
       => Reflect a ty where
   reflect :: a -> IO (J ty)
 
-#if ! __GLASGOW_HASKELL__ == 800
-foreign import ccall "wrapper" wrapFinalizer
-  :: (Ptr a -> IO ())
-  -> IO (FunPtr (Ptr a -> IO ()))
-
+#if ! (__GLASGOW_HASKELL__ == 800 && __GLASGOW_HASKELL_PATCHLEVEL1__ == 1)
 reifyMVector
   :: Storable a
   => (JArray ty -> IO (Ptr a))
@@ -331,8 +351,7 @@
 reifyMVector mk finalize jobj = do
     n <- getArrayLength jobj
     ptr <- mk jobj
-    ffinalize <- wrapFinalizer (finalize jobj)
-    fptr <- newForeignPtr ffinalize ptr
+    fptr <- newForeignPtr ptr (finalize jobj ptr)
     return (MVector.unsafeFromForeignPtr0 fptr (fromIntegral n))
 
 reflectMVector
@@ -483,7 +502,7 @@
 
 -- Instances can't be compiled on GHC 8.0.1 due to
 -- https://ghc.haskell.org/trac/ghc/ticket/12082.
-#if ! __GLASGOW_HASKELL__ == 800
+#if ! (__GLASGOW_HASKELL__ == 800 && __GLASGOW_HASKELL_PATCHLEVEL1__ == 1)
   type instance Interp (IOVector Int32) = 'Array ('Prim "int")
 
   instance Reify (IOVector Int32) ('Array ('Prim "int")) where