diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,12 @@
 # Changelog for the [`clash-lib`](http://hackage.haskell.org/package/clash-lib) package
 
-## 0.6.18
+## 0.6.19 *July 19th 2016*
+* Fixes bugs:
+  * Rounding error in `logBase` calculation
+
+## 0.6.18 *July 15th 2016*
+* New features:
+  * Better error location reporting
 * Fixes bugs:
   * Values of type Index 'n', where 'n' > 2^MACHINE_WIDTH, incorrectly considered non-synthesisable due to overflow
 
diff --git a/clash-lib.cabal b/clash-lib.cabal
--- a/clash-lib.cabal
+++ b/clash-lib.cabal
@@ -1,5 +1,5 @@
 Name:                 clash-lib
-Version:              0.6.18
+Version:              0.6.19
 Synopsis:             CAES Language for Synchronous Hardware - As a Library
 Description:
   CλaSH (pronounced ‘clash’) is a functional hardware description language that
@@ -101,6 +101,7 @@
                       filepath                >= 1.3.0.1  && < 1.5,
                       ghc                     >= 7.10.1   && < 8.1,
                       hashable                >= 1.2.1.0  && < 1.3,
+                      integer-gmp             >= 1.0      && < 1.1,
                       lens                    >= 3.9.2    && < 4.15,
                       mtl                     >= 2.1.2    && < 2.3,
                       pretty                  >= 1.1.1.0  && < 1.2,
diff --git a/src/CLaSH/Core/Util.hs b/src/CLaSH/Core/Util.hs
--- a/src/CLaSH/Core/Util.hs
+++ b/src/CLaSH/Core/Util.hs
@@ -407,10 +407,8 @@
       , length tys == 2
       , Right (Left i1) <- go (tys !! 0)
       , Right (Left i2) <- go (tys !! 1)
-      , i1 > 1
-      , i2 > 2
-      = return (Left (ceiling (logBase (fromIntegral i1 :: Double)
-                                       (fromIntegral i2 :: Double))))
+      , Just k <- clogBase i1 i2
+      = return (Left (toInteger k))
 
       | name2String tc == "GHC.TypeLits.Extra.GCD"
       , length tys == 2
diff --git a/src/CLaSH/Netlist/Util.hs b/src/CLaSH/Netlist/Util.hs
--- a/src/CLaSH/Netlist/Util.hs
+++ b/src/CLaSH/Netlist/Util.hs
@@ -186,19 +186,19 @@
 typeSize (BitVector i) = i
 typeSize (Index 0) = 0
 typeSize (Index 1) = 1
-typeSize (Index u) = clog2 u
+typeSize (Index u) = fromMaybe 0 (clogBase 2 u)
 typeSize (Signed i) = i
 typeSize (Unsigned i) = i
 typeSize (Vector n el) = n * typeSize el
 typeSize t@(SP _ cons) = conSize t +
   maximum (map (sum . map typeSize . snd) cons)
-typeSize (Sum _ dcs) = max 1 (clog2 . toInteger $ length dcs)
+typeSize (Sum _ dcs) = max 1 (fromMaybe 0 . clogBase 2 . toInteger $ length dcs)
 typeSize (Product _ tys) = sum $ map typeSize tys
 
 -- | Determines the bitsize of the constructor of a type
 conSize :: HWType
         -> Int
-conSize (SP _ cons) = clog2 . toInteger $ length cons
+conSize (SP _ cons) = fromMaybe 0 . clogBase 2 . toInteger $ length cons
 conSize t           = typeSize t
 
 -- | Gives the length of length-indexed types
diff --git a/src/CLaSH/Util.hs b/src/CLaSH/Util.hs
--- a/src/CLaSH/Util.hs
+++ b/src/CLaSH/Util.hs
@@ -8,6 +8,7 @@
 
 {-# LANGUAGE CPP                  #-}
 {-# LANGUAGE FlexibleInstances    #-}
+{-# LANGUAGE MagicHash            #-}
 {-# LANGUAGE Rank2Types           #-}
 {-# LANGUAGE TupleSections        #-}
 
@@ -36,6 +37,8 @@
 import Data.Version                   (Version)
 import Control.Lens
 import Debug.Trace                    (trace)
+import GHC.Base                       (Int(..),isTrue#,(==#),(+#))
+import GHC.Integer.Logarithms         (integerLogBase#)
 import qualified Language.Haskell.TH  as TH
 
 #ifdef CABAL
@@ -222,6 +225,12 @@
 clashLibVersion = error "development version"
 #endif
 
--- | ceiling (log_2(c))
-clog2 :: Integer -> Int
-clog2 = ceiling . logBase (2 :: Double) . fromIntegral
+-- | \x y -> ceiling (logBase x y), x > 1 && y > 0
+clogBase :: Integer -> Integer -> Maybe Int
+clogBase x y | x > 1 && y > 0 =
+  let z1 = integerLogBase# x y
+      z2 = integerLogBase# x (y-1)
+  in  if (isTrue# (z1 ==# z2))
+         then Just (I# (z1 +# 1#))
+         else Just (I# z1)
+clogBase _ _ = Nothing