diff --git a/roaring/roaring.go b/roaring/roaring.go
index 7b382ec6f..575515743 100644
--- a/roaring/roaring.go
+++ b/roaring/roaring.go
@@ -659,8 +659,9 @@ func (b *Bitmap) unionIntoTarget(target *Bitmap, others ...*Bitmap) {
 				// Once we've acquired a bitmap container (either by reusing the existing one
 				// or allocating a new one) then the last step is to iterate through all the
 				// other containers to see which ones have the same key, and union all of them
-				// into the target bitmap container.
-				for j, jIter := range bitmapIters {
+				// into the target bitmap container. Only need to loop starting from i because
+				// anything previous to that has already been handled.
+				for j, jIter := range bitmapIters[i:] {
 					jKey, jContainer := jIter.iter.Value()
 
 					if iKey == jKey {