diff --git a/index.d.ts b/index.d.ts
index 450a34c0a5..a3de6f0b6a 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -1,5 +1,5 @@
 // TypeScript Version: 3.0
-export type AxiosRequestHeaders = Record<string, string | number | boolean>;
+export type AxiosRequestHeaders = Record<string, string | string[] | number | boolean>;
 
 export type AxiosResponseHeaders = Record<string, string> & {
   "set-cookie"?: string[]