diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,3 +1,7 @@
+### 0.20.4
+
++ Better error diagnostics for [wrapObject](https://hackage.haskell.org/package/haskell-gi-base/docs/Data-GI-Base-ManagedPtr.html#v:wrapObject) and [newObject](https://hackage.haskell.org/package/haskell-gi-base/docs/Data-GI-Base-ManagedPtr.html#v:newObject).
+
 ### 0.20.3
 
 + Fixes for GHC 8.2.1 (and the corresponding `base-4.10.0`).
diff --git a/Data/GI/Base/BasicTypes.hs b/Data/GI/Base/BasicTypes.hs
--- a/Data/GI/Base/BasicTypes.hs
+++ b/Data/GI/Base/BasicTypes.hs
@@ -55,10 +55,6 @@
 import Foreign.Ptr (Ptr, FunPtr)
 import Foreign.ForeignPtr (ForeignPtr)
 
-#if MIN_VERSION_base(4,9,0)
-import GHC.TypeLits
-#endif
-
 import Data.GI.Base.CallStack (CallStack)
 import Data.GI.Base.GType
 
@@ -112,21 +108,6 @@
 class ManagedPtrNewtype a => GObject a where
     -- | The `GType` for this object.
     gobjectType :: a -> IO GType
-
--- Raise a more understandable type error whenever the `GObject a`
--- constraint is imposed on a type which has no such instance. This
--- helps in the common case where one passes a wrong type (such as
--- `Maybe Widget`) into a function with a `IsWidget a`
--- constraint. Without this type error, the resulting type error is
--- much less understandable, since GHC complains (at length) about a
--- missing type family instance for `ParentTypes`.
-#if MIN_VERSION_base(4,9,0)
-instance {-# OVERLAPPABLE #-}
-    (TypeError ('Text "Type ‘" ':<>: 'ShowType a ':<>:
-                'Text "’ does not descend from GObject."), ManagedPtrNewtype a)
-    => GObject a where
-    gobjectType = undefined
-#endif
 
 -- | A common omission in the introspection data is missing (nullable)
 -- annotations for return types, when they clearly are nullable. (A
diff --git a/Data/GI/Base/ManagedPtr.hs b/Data/GI/Base/ManagedPtr.hs
--- a/Data/GI/Base/ManagedPtr.hs
+++ b/Data/GI/Base/ManagedPtr.hs
@@ -218,19 +218,35 @@
 foreign import ccall "g_object_ref_sink" g_object_ref_sink ::
     Ptr a -> IO (Ptr a)
 
+-- | Print a warning when receiving a null pointer in a function that
+-- did not expect one, for easier debugging.
+nullPtrWarning :: String -> CallStack -> IO ()
+nullPtrWarning fn cs =
+  hPutStrLn stderr ("WARNING: Trying to wrap a null pointer in " ++ quotedFn
+                     ++ ", this may lead to crashes.\n\n"
+                     ++ "• Callstack for the unsafe call to "
+                     ++ quotedFn ++ ":\n"
+                     ++ prettyCallStack cs ++ "\n\n"
+                     ++ "This is probably a bug in the introspection data,\n"
+                     ++ "please report it at https://github.com/haskell-gi/haskell-gi/issues")
+  where quotedFn = "‘" ++ fn ++ "’"
+
 -- | Construct a Haskell wrapper for a 'GObject', increasing its
 -- reference count, or taking ownership of the floating reference if
 -- there is one.
-newObject :: (GObject a, GObject b) => (ManagedPtr a -> a) -> Ptr b -> IO a
+newObject :: (HasCallStack, GObject a, GObject b) =>
+             (ManagedPtr a -> a) -> Ptr b -> IO a
 newObject constructor ptr = do
+  when (ptr == nullPtr) (nullPtrWarning "newObject" callStack)
   void $ g_object_ref_sink ptr
   fPtr <- newManagedPtr' ptr_to_g_object_unref $ castPtr ptr
   return $! constructor fPtr
 
 -- | Same as 'newObject', but we steal ownership of the object.
-wrapObject :: forall a b. (GObject a, GObject b) =>
+wrapObject :: forall a b. (HasCallStack, GObject a, GObject b) =>
               (ManagedPtr a -> a) -> Ptr b -> IO a
 wrapObject constructor ptr = do
+  when (ptr == nullPtr) (nullPtrWarning "wrapObject" callStack)
   fPtr <- newManagedPtr' ptr_to_g_object_unref $ castPtr ptr
   return $! constructor fPtr
 
diff --git a/Data/GI/Base/Signals.hsc b/Data/GI/Base/Signals.hsc
--- a/Data/GI/Base/Signals.hsc
+++ b/Data/GI/Base/Signals.hsc
@@ -102,7 +102,7 @@
 -- | Same as `connectSignal`, specifying from the beginning that the
 -- handler is to be run before the default handler.
 --
--- > on = connectSignal SignalConnectBefore
+-- > on object signal handler = liftIO $ connectSignal signal object handler SignalConnectBefore
 on :: forall object info m.
       (GObject object, MonadIO m, SignalInfo info) =>
       object -> SignalProxy object info
@@ -111,7 +111,7 @@
 
 -- | Connect a signal to a handler, running the handler after the default one.
 --
--- > after = connectSignal SignalConnectAfter
+-- > after object signal handler = liftIO $ connectSignal signal object handler SignalConnectAfter
 after :: forall object info m.
       (GObject object, MonadIO m, SignalInfo info) =>
       object -> SignalProxy object info
diff --git a/haskell-gi-base.cabal b/haskell-gi-base.cabal
--- a/haskell-gi-base.cabal
+++ b/haskell-gi-base.cabal
@@ -1,5 +1,5 @@
 name:                haskell-gi-base
-version:             0.20.3
+version:             0.20.4
 synopsis:            Foundation for libraries generated by haskell-gi
 description:         Foundation for libraries generated by haskell-gi
 homepage:            https://github.com/haskell-gi/haskell-gi-base