diff --git a/graph/iterator/optional_iterator.go b/graph/iterator/optional_iterator.go
index c8c7fec..84e7154 100644
--- a/graph/iterator/optional_iterator.go
+++ b/graph/iterator/optional_iterator.go
@@ -156,3 +156,5 @@ func (it *Optional) Stats() graph.IteratorStats {
 func (it *Optional) Size() (int64, bool) {
 	return 0, true
 }
+
+var _ graph.Iterator = &Optional{}