diff --git a/VERSION b/VERSION
--- a/VERSION
+++ b/VERSION
@@ -1,3 +1,5 @@
+0.13.1 (2015-03-10):
+    - Fixed a major bug in sum
 0.13 (2015-02-17):
     - monomorphized logFloat, logToLogFloat, fromLogFloat, logFromLogFloat
     - added sum, product, pow
diff --git a/logfloat.cabal b/logfloat.cabal
--- a/logfloat.cabal
+++ b/logfloat.cabal
@@ -1,5 +1,5 @@
 ----------------------------------------------------------------
--- wren gayle romano <wren@community.haskell.org>   ~ 2015.02.17
+-- wren gayle romano <wren@community.haskell.org>   ~ 2015.03.10
 ----------------------------------------------------------------
 
 -- By and large Cabal >=1.2 is fine; but >= 1.6 gives tested-with:
@@ -8,7 +8,7 @@
 Build-Type:     Simple
 
 Name:           logfloat
-Version:        0.13.0.1
+Version:        0.13.1
 Stability:      experimental
 Homepage:       http://code.haskell.org/~wren/
 Author:         wren gayle romano
diff --git a/src/Data/Number/LogFloat.hs b/src/Data/Number/LogFloat.hs
--- a/src/Data/Number/LogFloat.hs
+++ b/src/Data/Number/LogFloat.hs
@@ -512,12 +512,12 @@
 --
 -- /Since: 0.13/
 sum :: [LogFloat] -> LogFloat
-sum xs = LogFloat (theMax + theSum)
+sum xs = LogFloat (theMax + log theSum)
     where
-    theMax = logFromLogFloat (maximum xs)
+    LogFloat theMax = maximum xs
     
     -- compute @\log \sum_{x \in xs} \exp(x - theMax)@
-    theSum = foldl' (\ acc x -> acc + exp (logFromLogFloat x - theMax)) 0 xs
+    theSum = foldl' (\ acc (LogFloat x) -> acc + exp (x - theMax)) 0 xs
 
 -- TODO: expose a single-pass version for the special case where
 -- the first element of the list is (promised to be) the maximum