diff --git a/CHANGES.md b/CHANGES.md
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,6 +1,11 @@
 Changes
 =======
 
+Version 1.0.3
+-------------
+
+Fix a bug where no test cases were generated for some functional types (#19).
+
 Version 1.0.2
 -------------
 
diff --git a/Test/SmallCheck.hs b/Test/SmallCheck.hs
--- a/Test/SmallCheck.hs
+++ b/Test/SmallCheck.hs
@@ -14,6 +14,7 @@
 -- to the README at
 -- <https://github.com/feuerbach/smallcheck/blob/master/README.md>
 --------------------------------------------------------------------
+{-# LANGUAGE Safe #-}
 module Test.SmallCheck (
   -- * Constructing tests
 
diff --git a/Test/SmallCheck/Property.hs b/Test/SmallCheck/Property.hs
--- a/Test/SmallCheck/Property.hs
+++ b/Test/SmallCheck/Property.hs
@@ -9,6 +9,9 @@
 --
 -- Properties and tools to construct them.
 --------------------------------------------------------------------
+{-# LANGUAGE Trustworthy #-}
+  -- Trustworthy is needed because of the hand-written Typeable instance.
+  -- Kind-polymorphic Typeable will solve this.
 {-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, TypeFamilies,
              ScopedTypeVariables #-}
 module Test.SmallCheck.Property (
diff --git a/Test/SmallCheck/Series.hs b/Test/SmallCheck/Series.hs
--- a/Test/SmallCheck/Series.hs
+++ b/Test/SmallCheck/Series.hs
@@ -139,7 +139,8 @@
   --
   -- If @d > 0@, these functions inspect each of their arguments up to the depth
   -- @d-1@ (as defined by the 'coseries' functions for the corresponding
-  -- types) and return values produced by @s@.
+  -- types) and return values produced by @s@. The depth to which the
+  -- values are enumerated does not depend on the depth of inspection.
 
   alts0, alts1, alts2, alts3, alts4, newtypeAlts,
 
@@ -254,12 +255,17 @@
 constM :: Monad m => m b -> m (a -> b)
 constM = liftM const
 
+-- | Fix the depth of a series at the current level. The resulting series
+-- will no longer depend on the \"ambient\" depth.
+fixDepth :: Series m a -> Series m (Series m a)
+fixDepth s = getDepth >>= \d -> return $ localDepth (const d) s
+
 -- | If the current depth is 0, evaluate the first argument. Otherwise,
 -- evaluate the second argument with decremented depth.
 decDepthChecked :: Series m a -> Series m a -> Series m a
 decDepthChecked b r = do
   d <- getDepth
-  if d == 0
+  if d <= 0
     then b
     else decDepth r
 
@@ -307,27 +313,31 @@
 alts0 s = s
 
 alts1 :: CoSerial m a => Series m b -> Series m (a->b)
-alts1 rs =
+alts1 rs = do
+  rs <- fixDepth rs
   decDepthChecked (constM rs) (coseries rs)
 
 alts2
   :: (CoSerial m a, CoSerial m b)
   => Series m c -> Series m (a->b->c)
-alts2 rs =
+alts2 rs = do
+  rs <- fixDepth rs
   decDepthChecked
     (constM $ constM rs)
     (coseries $ coseries rs)
 
 alts3 ::  (CoSerial m a, CoSerial m b, CoSerial m c) =>
             Series m d -> Series m (a->b->c->d)
-alts3 rs =
+alts3 rs = do
+  rs <- fixDepth rs
   decDepthChecked
     (constM $ constM $ constM rs)
     (coseries $ coseries $ coseries rs)
 
 alts4 ::  (CoSerial m a, CoSerial m b, CoSerial m c, CoSerial m d) =>
             Series m e -> Series m (a->b->c->d->e)
-alts4 rs =
+alts4 rs = do
+  rs <- fixDepth rs
   decDepthChecked
     (constM $ constM $ constM $ constM rs)
     (coseries $ coseries $ coseries $ coseries rs)
@@ -436,8 +446,13 @@
 instance (Integral a, Serial m a) => Serial m (N a) where
   series = generate $ \d -> map (N . fromIntegral) [0..d]
 
-instance (Integral a, Serial m a) => CoSerial m (N a) where
+instance (Integral a, Monad m) => CoSerial m (N a) where
   coseries rs =
+    -- This is a recursive function, because @alts1 rs@ typically calls
+    -- back to 'coseries' (but with lower depth).
+    --
+    -- The recursion stops when depth == 0. Then alts1 produces a constant
+    -- function, and doesn't call back to 'coseries'.
     alts0 rs >>- \z ->
     alts1 rs >>- \f ->
     return $ \(N i) ->
diff --git a/Test/SmallCheck/SeriesMonad.hs b/Test/SmallCheck/SeriesMonad.hs
--- a/Test/SmallCheck/SeriesMonad.hs
+++ b/Test/SmallCheck/SeriesMonad.hs
@@ -1,3 +1,4 @@
+{-# LANGUAGE Trustworthy #-} -- GeneralizedNewtypeDeriving
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}
 module Test.SmallCheck.SeriesMonad where
 
diff --git a/smallcheck.cabal b/smallcheck.cabal
--- a/smallcheck.cabal
+++ b/smallcheck.cabal
@@ -1,5 +1,5 @@
 Name:          smallcheck
-Version:       1.0.2
+Version:       1.0.3
 Cabal-Version: >= 1.6
 License:       BSD3
 License-File:  LICENSE
@@ -27,7 +27,7 @@
 Source-repository this
   type:     git
   location: git://github.com/feuerbach/smallcheck.git
-  tag:      v1.0.2
+  tag:      v1.0.3
 
 Library