diff --git a/index.d.ts b/index.d.ts
index 8ab8e51..54610e3 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -6,7 +6,7 @@ export type AxiosResponseHeaders = Record<string, string> & {
 };
 
 export interface AxiosRequestTransformer {
-  (data: any, headers?: AxiosRequestHeaders): any;
+  (data: any, headers: AxiosRequestHeaders): any;
 }
 
 export interface AxiosResponseTransformer {