diff --git a/index.d.ts b/index.d.ts
index 452d2cc273..7196a3e352 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -111,7 +111,7 @@ export interface SerializerOptions {
   visitor?: SerializerVisitor;
   dots?: boolean;
   metaTokens?: boolean;
-  indexes?: boolean;
+  indexes?: boolean | null;
 }
 
 // tslint:disable-next-line
diff --git a/test/typescript/axios.ts b/test/typescript/axios.ts
index 3b9b4b7adb..2bfb23c07a 100644
--- a/test/typescript/axios.ts
+++ b/test/typescript/axios.ts
@@ -274,6 +274,17 @@ instance1.defaults.timeout = 2500;
 
 axios.create({ headers: { foo: 'bar' } });
 axios.create({ headers: { common: { foo: 'bar' } } });
+axios.create({
+  headers: {
+    'Content-Type': 'application/json',
+  },
+  formSerializer: {
+    indexes: null,
+  },
+  paramsSerializer: {
+    indexes: null,
+  },
+});
 
 // Interceptors