diff --git a/Control/DeepSeq.hs b/Control/DeepSeq.hs
index 497bbb4..3a1d473 100644
--- a/Control/DeepSeq.hs
+++ b/Control/DeepSeq.hs
@@ -107,6 +107,12 @@ import Foreign.C.Types
 import System.Exit ( ExitCode(..) )
 import System.Mem.StableName ( StableName )
 
+#if MIN_VERSION_base(4,8,0)
+import Foreign.ForeignPtr
+#else
+import Foreign.ForeignPtr.Safe
+#endif
+
 #if MIN_VERSION_base(4,6,0)
 import Data.Ord ( Down(Down) )
 #else
@@ -125,6 +131,10 @@ import Data.Type.Equality ( (:~:) )
 import Control.DeepSeq.BackDoor ( (:~:) )
 #endif
 
+#if MIN_VERSION_base(4,0,0)
+import Control.DeepSeq.BackDoor ( TVar )
+#endif
+
 #if MIN_VERSION_base(4,8,0)
 import Data.Functor.Identity ( Identity(..) )
 import Data.Typeable ( rnfTypeRep, rnfTyCon )
@@ -627,7 +637,7 @@ instance (Ix i, NFData i, NFData e) => NFData (UArray i e) where
 instance (NFData i) => NFData1 (UArray i) where
 #else
 -- |@since 1.4.5.0
-instance (Ix i, NFData i) => NFData1 (UArray a) where
+instance (Ix i, NFData i) => NFData1 (UArray i) where
 #endif
     liftRnf _ = rwhnf
 
@@ -752,6 +762,16 @@ instance NFData (MVar a) where
 instance NFData1 MVar where
     liftRnf _ = rwhnf
 
+#if MIN_VERSION_base(4,0,0)
+-- | __NOTE__: Only strict in the reference and not the referenced value.
+--
+-- @since 1.4.5.0
+instance NFData (TVar a) where
+    rnf = rwhnf
+instance NFData1 TVar where
+    liftRnf _ = rwhnf
+#endif
+
 ----------------------------------------------------------------------------
 -- GHC Specifics
 
diff --git a/Control/DeepSeq/BackDoor.hs b/Control/DeepSeq/BackDoor.hs
index 343ec56..d9347a7 100644
--- a/Control/DeepSeq/BackDoor.hs
+++ b/Control/DeepSeq/BackDoor.hs
@@ -5,14 +5,6 @@
 -- This module only re-export reasonably safe entities from non-safe
 -- modules when there is no safe alternative
 
-#if MIN_VERSION_base(4,9,0) || (MIN_VERSION_base(4,6,0) && !MIN_VERSION_base(4,7,0))
-{-# LANGUAGE Safe #-}
-
-module Control.DeepSeq.BackDoor
-       {-# WARNING "This module is empty! Do not import me!" #-}
-       () where
-
-#else
 {-# LANGUAGE Trustworthy #-}
 
 module Control.DeepSeq.BackDoor
@@ -20,14 +12,17 @@ module Control.DeepSeq.BackDoor
     ) where
 
 #if !(MIN_VERSION_base(4,6,0))
--- not SAFE
+-- not safe, but Down is in Data.Ord starting with base-4.6
 import GHC.Exts as X ( Down(Down) )
 #endif
 
-#if MIN_VERSION_base(4,10,0)
--- Data.Type.Equality SAFE starting with base-4.10
+#if MIN_VERSION_base(4,9,0)
+-- Data.Type.Equality present from base-4.7, trustworthy starting with base-4.9
 #elif MIN_VERSION_base(4,7,0)
 import Data.Type.Equality as X ( (:~:) )
 #endif
 
+#if MIN_VERSION_base(4,0,0)
+-- present since at least base-4.0, maybe sooner?
+import GHC.Conc as X ( TVar(TVar) )
 #endif