diff --git a/packages/agent-harness/src/index.ts b/packages/agent-harness/src/index.ts
index b9c1683..b737e0b 100644
--- a/packages/agent-harness/src/index.ts
+++ b/packages/agent-harness/src/index.ts
@@ -361,7 +361,7 @@ const SlackOAuthResponseSchema = z.object({
   app_id: z.string().optional(),
   bot_user_id: z.string().optional(),
   team: z.object({ id: z.string().min(1), name: z.string().optional() }),
-  enterprise: z.object({ id: z.string().optional() }).optional(),
+  enterprise: z.object({ id: z.string().optional() }).nullish(),
   scope: z.string().optional(),
 });
 
diff --git a/packages/agent-harness/src/testing/fake-slack-server.ts b/packages/agent-harness/src/testing/fake-slack-server.ts
index 2fac3d7..c2445cd 100644
--- a/packages/agent-harness/src/testing/fake-slack-server.ts
+++ b/packages/agent-harness/src/testing/fake-slack-server.ts
@@ -65,6 +65,7 @@ export class FakeSlackServer {
               id: this.options.teamId,
               name: this.options.teamName,
             },
+            enterprise: null,
             scope: scopes.join(","),
           }),
         );