diff --git a/generator/src/main/resources/line-bot-sdk-nodejs-generator/apiBody/multipart.pebble b/generator/src/main/resources/line-bot-sdk-nodejs-generator/apiBody/multipart.pebble
index a36b0251d..f1bfe6c7a 100644
--- a/generator/src/main/resources/line-bot-sdk-nodejs-generator/apiBody/multipart.pebble
+++ b/generator/src/main/resources/line-bot-sdk-nodejs-generator/apiBody/multipart.pebble
@@ -8,11 +8,17 @@
 {% endif -%}
         const form = new FormData();
 {% for param in op.formParams -%}
-        {% if param.isFile -%}
-        form.append("{{param.paramName}}", {{param.paramName}}); // file
-        {% else -%}
-        form.append("{{param.paramName}}", String({{param.paramName}}));
-        {% endif -%}
+{% if not param.required -%}
+        if ({{ param.paramName }} !== undefined) {
+{% endif -%}
+{% if param.isFile -%}
+          form.append("{{ param.baseName }}", {{ param.paramName }}); // file
+{% else -%}
+          form.append("{{ param.baseName }}", String({{ param.paramName }}));
+{% endif -%}
+{% if not param.required -%}
+        }
+{% endif -%}
 {% endfor %}
         const res = await this.httpClient.{{op.httpMethod|lower}}{% if op.hasFormParams %}Form{% endif %}Multipart(
             {% if op.pathParams|length > 0 %}requestPath{% else %}"{{ op.path }}"{% endif %},
diff --git a/lib/manage-audience/api/manageAudienceBlobClient.ts b/lib/manage-audience/api/manageAudienceBlobClient.ts
index 6f5355020..20db329db 100644
--- a/lib/manage-audience/api/manageAudienceBlobClient.ts
+++ b/lib/manage-audience/api/manageAudienceBlobClient.ts
@@ -114,8 +114,12 @@ export class ManageAudienceBlobClient {
     uploadDescription?: string,
   ): Promise<Types.ApiResponseType<Types.MessageAPIResponseBase>> {
     const form = new FormData();
-    form.append("audienceGroupId", String(audienceGroupId));
-    form.append("uploadDescription", String(uploadDescription));
+    if (audienceGroupId !== undefined) {
+      form.append("audienceGroupId", String(audienceGroupId));
+    }
+    if (uploadDescription !== undefined) {
+      form.append("uploadDescription", String(uploadDescription));
+    }
     form.append("file", file); // file
 
     const res = await this.httpClient.putFormMultipart(
@@ -171,9 +175,15 @@ export class ManageAudienceBlobClient {
     uploadDescription?: string,
   ): Promise<Types.ApiResponseType<CreateAudienceGroupResponse>> {
     const form = new FormData();
-    form.append("description", String(description));
-    form.append("isIfaAudience", String(isIfaAudience));
-    form.append("uploadDescription", String(uploadDescription));
+    if (description !== undefined) {
+      form.append("description", String(description));
+    }
+    if (isIfaAudience !== undefined) {
+      form.append("isIfaAudience", String(isIfaAudience));
+    }
+    if (uploadDescription !== undefined) {
+      form.append("uploadDescription", String(uploadDescription));
+    }
     form.append("file", file); // file
 
     const res = await this.httpClient.postFormMultipart(
diff --git a/test/libs-manageAudience.spec.ts b/test/libs-manageAudience.spec.ts
index 29046230e..e13193b61 100644
--- a/test/libs-manageAudience.spec.ts
+++ b/test/libs-manageAudience.spec.ts
@@ -1,7 +1,7 @@
 import { manageAudience } from "../lib/index.js";
 import { http, HttpResponse } from "msw";
 import { setupServer } from "msw/node";
-import { deepEqual, equal, match } from "node:assert";
+import { deepEqual, equal, match, ok, strictEqual } from "node:assert";
 
 import { describe, it, beforeAll, afterAll, afterEach } from "vitest";
 
@@ -129,4 +129,74 @@ describe("manageAudience", () => {
     equal(requestCount, 1);
     deepEqual(res.body, {});
   });
+
+  it("createAudienceForUploadingUserIds omits undefined optional fields", async () => {
+    let received: FormData | null = null;
+    server.use(
+      http.post(
+        "https://api-data.line.me/v2/bot/audienceGroup/upload/byFile",
+        async ({ request }) => {
+          received = await request.formData();
+          return HttpResponse.json({});
+        },
+      ),
+    );
+
+    const file = new Blob(["user-id-1"], { type: "text/plain" });
+    await blobClient.createAudienceForUploadingUserIds(file);
+
+    const form: FormData = received!;
+    ok(form);
+    ok(form.get("file") instanceof File);
+    strictEqual(form.has("description"), false);
+    strictEqual(form.has("isIfaAudience"), false);
+    strictEqual(form.has("uploadDescription"), false);
+  });
+
+  it("addUserIdsToAudience omits undefined optional fields", async () => {
+    let received: FormData | null = null;
+    server.use(
+      http.put(
+        "https://api-data.line.me/v2/bot/audienceGroup/upload/byFile",
+        async ({ request }) => {
+          received = await request.formData();
+          return HttpResponse.json({});
+        },
+      ),
+    );
+
+    const file = new Blob(["user-id-1"], { type: "text/plain" });
+    await blobClient.addUserIdsToAudience(file);
+
+    const form: FormData = received!;
+    ok(form);
+    ok(form.get("file") instanceof File);
+    strictEqual(form.has("audienceGroupId"), false);
+    strictEqual(form.has("uploadDescription"), false);
+  });
+
+  it("createAudienceForUploadingUserIds preserves false for isIfaAudience", async () => {
+    let received: FormData | null = null;
+    server.use(
+      http.post(
+        "https://api-data.line.me/v2/bot/audienceGroup/upload/byFile",
+        async ({ request }) => {
+          received = await request.formData();
+          return HttpResponse.json({});
+        },
+      ),
+    );
+
+    const file = new Blob(["user-id-1"], { type: "text/plain" });
+    await blobClient.createAudienceForUploadingUserIds(
+      file,
+      "test_description",
+      false,
+    );
+
+    const form: FormData = received!;
+    ok(form);
+    strictEqual(form.get("isIfaAudience"), "false");
+    strictEqual(form.get("description"), "test_description");
+  });
 });