diff --git a/packages/core/src/runtime/actor.ts b/packages/core/src/runtime/actor.ts
index b902426..e234370 100644
--- a/packages/core/src/runtime/actor.ts
+++ b/packages/core/src/runtime/actor.ts
@@ -371,7 +371,10 @@ export class Turn {
         f.actor.dataflowGraph.withSubject(b, b);
     }
 
-    assertDataflow(assertionFunction: () => {target: Ref, assertion: Assertion}) {
+    assertDataflow(assertionFunction: () => {
+        target: Ref | undefined,
+        assertion: Assertion | undefined
+    }) {
         let handle: Handle | undefined = void 0;
         let target: Ref | undefined = void 0;
         let assertion: Assertion | undefined = void 0;