diff --git a/lib/Graphable.ts b/lib/Graphable.ts
index 4450029..fc4f3b5 100644
--- a/lib/Graphable.ts
+++ b/lib/Graphable.ts
@@ -1,5 +1,5 @@
 import { Graph } from './Graph';
 
 export interface Graphable {
-  to_graph(g: Graph, ...args: unknown[]): void;
+  to_graph(g: Graph, ...args: unknown[]): unknown;
 }