diff --git a/lib/uint64set/uint64set.go b/lib/uint64set/uint64set.go
index 5132c6e27a..54cc96b870 100644
--- a/lib/uint64set/uint64set.go
+++ b/lib/uint64set/uint64set.go
@@ -31,7 +31,8 @@ func (s *bucket32Sorter) Swap(i, j int) {
 // Clone returns an independent copy of s.
 func (s *Set) Clone() *Set {
 	if s == nil {
-		return nil
+		// Return an empty set, so data could be added into it later.
+		return &Set{}
 	}
 	var dst Set
 	dst.itemsCount = s.itemsCount