diff --git a/src/browser/run/runner.ts b/src/browser/run/runner.ts
index 0cd8eb9..a37ccb9 100644
--- a/src/browser/run/runner.ts
+++ b/src/browser/run/runner.ts
@@ -446,7 +446,7 @@ export async function runBrowserProgram(
             rejectRun = undefined;
           }
           try {
-            const serialized = JSON.stringify(value);
+            const serialized = JSON.stringify(value === undefined ? null : value);
             if (serialized === undefined) throw new TypeError('Result is not JSON serializable.');
             return serialized;
           } catch (cause) {