diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,9 @@
 # Changelog for the [`ghc-typelits-natnormalise`](http://hackage.haskell.org/package/ghc-typelits-natnormalise) package
 
+## 0.4.5 *July 20th 2016*
+* Fixes bugs:
+  * Reifying negative exponent causes GHC panic
+
 ## 0.4.4 *July 19th 2016*
 * Fixes bugs:
   * Rounding error in `logBase` calculation
diff --git a/ghc-typelits-natnormalise.cabal b/ghc-typelits-natnormalise.cabal
--- a/ghc-typelits-natnormalise.cabal
+++ b/ghc-typelits-natnormalise.cabal
@@ -1,5 +1,5 @@
 name:                ghc-typelits-natnormalise
-version:             0.4.4
+version:             0.4.5
 synopsis:            GHC typechecker plugin for types of kind GHC.TypeLits.Nat
 description:
   A type checker plugin for GHC that can solve /equalities/ of types of kind
diff --git a/src/GHC/TypeLits/Normalise/Unify.hs b/src/GHC/TypeLits/Normalise/Unify.hs
--- a/src/GHC/TypeLits/Normalise/Unify.hs
+++ b/src/GHC/TypeLits/Normalise/Unify.hs
@@ -107,14 +107,37 @@
                                  p
 
 reifyProduct :: CoreProduct -> Type
-reifyProduct = foldr1 (\t1 t2 -> mkTyConApp typeNatMulTyCon [t1,t2])
-             . map reifySymbol . unP
+reifyProduct (P ps) =
+    let ps' = map reifySymbol (foldr mergeExp [] ps)
+    in  foldr (\t1 t2 -> mkTyConApp typeNatMulTyCon [t1,t2]) (head ps') (tail ps')
+  where
+    -- "2 ^ -1 * 2 ^ a" must be merged into "2 ^ (a-1)", otherwise GHC barfs
+    -- at the "2 ^ -1" because of the negative exponent.
+    mergeExp :: CoreSymbol -> [Either CoreSymbol (CoreSOP,CoreProduct,Integer)]
+                           -> [Either CoreSymbol (CoreSOP,CoreProduct,Integer)]
+    mergeExp (E s1 (P [I i])) (y:ys)
+      | i < 0
+      , Left (E s2 p2) <- y
+      , s1 == s2
+      = Right (s1,p2,negate i) : ys
+      | i < 0
+      , Right (s2,p2,j) <- y
+      , s1 == s2
+      = Right (s1,p2,j+negate i) : ys
+    mergeExp x ys = Left x:ys
 
-reifySymbol :: CoreSymbol -> Type
-reifySymbol (I i)   = mkNumLitTy i
-reifySymbol (C c)   = c
-reifySymbol (V v)   = mkTyVarTy v
-reifySymbol (E s p) = mkTyConApp typeNatExpTyCon [reifySOP s,reifyProduct p]
+reifySymbol :: Either CoreSymbol (CoreSOP,CoreProduct,Integer) -> Type
+reifySymbol (Left (I i)  )  = mkNumLitTy i
+reifySymbol (Left (C c)  )  = c
+reifySymbol (Left (V v)  )  = mkTyVarTy v
+reifySymbol (Left (E s p))  = mkTyConApp typeNatExpTyCon [reifySOP s,reifyProduct p]
+reifySymbol (Right (s,p,i)) = mkTyConApp typeNatExpTyCon
+                                         [reifySOP s
+                                         ,mkTyConApp typeNatSubTyCon
+                                                     [reifyProduct p
+                                                     ,mkNumLitTy i
+                                                     ]
+                                         ]
 
 -- | A substitution is essentially a list of (variable, 'SOP') pairs,
 -- but we keep the original 'Ct' that lead to the substitution being