diff --git a/packages/core/src/runtime/actor.ts b/packages/core/src/runtime/actor.ts
index 4d82298..1c2913b 100644
--- a/packages/core/src/runtime/actor.ts
+++ b/packages/core/src/runtime/actor.ts
@@ -80,7 +80,10 @@ export function toRef(_v: any): Ref | undefined {
     return isRef(_v) ? _v : void 0;
 }
 
-export function assertionFrom(a: Assertable): Assertion {
+export function assertionFrom(a: Assertable): Assertion;
+export function assertionFrom(a: Assertable | undefined): Assertion | undefined;
+export function assertionFrom(a: Assertable | undefined): Assertion | undefined {
+    if (a === void 0) return void 0;
     if (typeof a === 'object' && '__as_preserve__' in a) {
         return fromJS(a);
     } else {
@@ -464,6 +467,7 @@ export class Turn {
         let assertion: Assertable | undefined = void 0;
         this.dataflow(() => {
             let {target: nextTarget, assertion: nextAssertion} = assertionFunction();
+            nextAssertion = assertionFrom(nextAssertion);
             if (target !== nextTarget || !is(assertion, nextAssertion)) {
                 target = nextTarget;
                 assertion = nextAssertion;