diff --git a/modal-js/src/sandbox.ts b/modal-js/src/sandbox.ts
index 1f9961ec..6810d151 100644
--- a/modal-js/src/sandbox.ts
+++ b/modal-js/src/sandbox.ts
@@ -764,7 +764,7 @@ export class Sandbox {
       }
       if (resp.taskResult) {
         throw new Error(
-          `Sandbox ${this.sandboxId} has already completed with result: ${resp.taskResult}`,
+          `Sandbox ${this.sandboxId} has already completed with result: ${JSON.stringify(resp.taskResult)}`,
         );
       }
       this.#taskId = resp.taskId;