diff --git a/roaring/containers_slice.go b/roaring/containers_slice.go
index 67e936c24..610fd43a2 100644
--- a/roaring/containers_slice.go
+++ b/roaring/containers_slice.go
@@ -212,7 +212,7 @@ func (sc *sliceContainers) Update(key uint64, fn func(*Container, bool) (*Contai
 		// don't expand the slice just to add a nil container, we
 		// could return that anyway
 		if write && nc != nil {
-			sc.insertAt(key, nc, -i-1)
+			sc.insertAt(key, nc, i)
 		}
 	}
 }