diff --git a/parallel.go b/parallel.go
index 6d8e2936..f1d08a99 100644
--- a/parallel.go
+++ b/parallel.go
@@ -357,7 +357,7 @@ func ParOr(parallelism int, bitmaps ...*Bitmap) *Bitmap {
 		return bitmaps[0]
 	}
 
-	keyRange := hKey - lKey + 1
+	keyRange := int(hKey) - int(lKey) + 1
 	if keyRange == 1 {
 		// revert to FastOr. Since the key range is 0
 		// no container-level aggregation parallelism is achievable