1 files changed, 1 insertions, 1 deletions

diff --git a/src/Data/Word64RangeMap.hs b/src/Data/Word64RangeMap.hs
index 36428233..0b25f56d 100644
--- a/src/Data/Word64RangeMap.hs
+++ b/src/Data/Word64RangeMap.hs
@@ -96,7 +96,7 @@ lookupArray i r = do
     ra <- readRefArray r
     (zr,nr) <- getBounds ra -- bounds of mutable range array
     let (dr,mr) = (nr+1 - zr) `divMod` 2
-    lookupArrayHelper i r (zr+dr+mr) zr nr
+    lookupArrayHelper i r (zr+dr+mr-1) zr nr
 
 lookupArrayHelper :: RangeArray ra m b ref => Index -> RangeMap ra b ref
                                        -> OuterIndex{- current position  -}