diff --git a/index.d.cts b/index.d.cts
index 5dcbc380..e91217b7 100644
--- a/index.d.cts
+++ b/index.d.cts
@@ -10,13 +10,6 @@ type MethodsHeaders = Partial<{
 
 type AxiosHeaderMatcher = (this: AxiosHeaders, value: string, name: string, headers: RawAxiosHeaders) => boolean;
 
-type AxiosHeaderSetter = (value: AxiosHeaderValue, rewrite?: boolean | AxiosHeaderMatcher) => AxiosHeaders;
-
-type AxiosHeaderGetter = ((parser?: RegExp) => RegExpExecArray | null) |
-    ((matcher?: AxiosHeaderMatcher) => AxiosHeaderValue);
-
-type AxiosHeaderTester = (matcher?: AxiosHeaderMatcher) => boolean;
-
 declare class AxiosHeaders {
   constructor(
       headers?: RawAxiosHeaders | AxiosHeaders
@@ -38,7 +31,7 @@ declare class AxiosHeaders {
 
   normalize(format: boolean): AxiosHeaders;
 
-  concat(...targets: Array<AxiosHeaders | RawAxiosHeaders | string>): AxiosHeaders;
+  concat(...targets: Array<AxiosHeaders | RawAxiosHeaders | string | undefined | null>): AxiosHeaders;
 
   toJSON(asStrings?: boolean): RawAxiosHeaders;
 
@@ -46,27 +39,37 @@ declare class AxiosHeaders {
 
   static accessor(header: string | string[]): AxiosHeaders;
 
-  static concat(...targets: Array<AxiosHeaders | RawAxiosHeaders | string>): AxiosHeaders;
+  static concat(...targets: Array<AxiosHeaders | RawAxiosHeaders | string | undefined | null>): AxiosHeaders;
 
-  setContentType: AxiosHeaderSetter;
-  getContentType: AxiosHeaderGetter;
-  hasContentType: AxiosHeaderTester;
+  setContentType(value: AxiosHeaderValue, rewrite?: boolean | AxiosHeaderMatcher): AxiosHeaders;
+  getContentType(parser?: RegExp): RegExpExecArray | null;
+  getContentType(matcher?: AxiosHeaderMatcher): AxiosHeaderValue;
+  hasContentType(matcher?: AxiosHeaderMatcher): boolean;
 
-  setContentLength: AxiosHeaderSetter;
-  getContentLength: AxiosHeaderGetter;
-  hasContentLength: AxiosHeaderTester;
+  setContentLength(value: AxiosHeaderValue, rewrite?: boolean | AxiosHeaderMatcher): AxiosHeaders;
+  getContentLength(parser?: RegExp): RegExpExecArray | null;
+  getContentLength(matcher?: AxiosHeaderMatcher): AxiosHeaderValue;
+  hasContentLength(matcher?: AxiosHeaderMatcher): boolean;
 
-  setAccept: AxiosHeaderSetter;
-  getAccept: AxiosHeaderGetter;
-  hasAccept: AxiosHeaderTester;
+  setAccept(value: AxiosHeaderValue, rewrite?: boolean | AxiosHeaderMatcher): AxiosHeaders;
+  getAccept(parser?: RegExp): RegExpExecArray | null;
+  getAccept(matcher?: AxiosHeaderMatcher): AxiosHeaderValue;
+  hasAccept(matcher?: AxiosHeaderMatcher): boolean;
 
-  setUserAgent: AxiosHeaderSetter;
-  getUserAgent: AxiosHeaderGetter;
-  hasUserAgent: AxiosHeaderTester;
+  setUserAgent(value: AxiosHeaderValue, rewrite?: boolean | AxiosHeaderMatcher): AxiosHeaders;
+  getUserAgent(parser?: RegExp): RegExpExecArray | null;
+  getUserAgent(matcher?: AxiosHeaderMatcher): AxiosHeaderValue;
+  hasUserAgent(matcher?: AxiosHeaderMatcher): boolean;
 
-  setContentEncoding: AxiosHeaderSetter;
-  getContentEncoding: AxiosHeaderGetter;
-  hasContentEncoding: AxiosHeaderTester;
+  setContentEncoding(value: AxiosHeaderValue, rewrite?: boolean | AxiosHeaderMatcher): AxiosHeaders;
+  getContentEncoding(parser?: RegExp): RegExpExecArray | null;
+  getContentEncoding(matcher?: AxiosHeaderMatcher): AxiosHeaderValue;
+  hasContentEncoding(matcher?: AxiosHeaderMatcher): boolean;
+
+  setAuthorization(value: AxiosHeaderValue, rewrite?: boolean | AxiosHeaderMatcher): AxiosHeaders;
+  getAuthorization(parser?: RegExp): RegExpExecArray | null;
+  getAuthorization(matcher?: AxiosHeaderMatcher): AxiosHeaderValue;
+  hasAuthorization(matcher?: AxiosHeaderMatcher): boolean;
 
   [Symbol.iterator](): IterableIterator<[string, AxiosHeaderValue]>;
 }
diff --git a/index.d.ts b/index.d.ts
index 4cde8ff0..0249403c 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -11,13 +11,6 @@ type MethodsHeaders = Partial<{
 
 type AxiosHeaderMatcher = (this: AxiosHeaders, value: string, name: string, headers: RawAxiosHeaders) => boolean;
 
-type AxiosHeaderSetter = (value: AxiosHeaderValue, rewrite?: boolean | AxiosHeaderMatcher) => AxiosHeaders;
-
-type AxiosHeaderGetter = ((parser?: RegExp) => RegExpExecArray | null) |
-    ((matcher?: AxiosHeaderMatcher) => AxiosHeaderValue);
-
-type AxiosHeaderTester = (matcher?: AxiosHeaderMatcher) => boolean;
-
 export class AxiosHeaders {
   constructor(
       headers?: RawAxiosHeaders | AxiosHeaders
@@ -39,7 +32,7 @@ export class AxiosHeaders {
 
   normalize(format: boolean): AxiosHeaders;
 
-  concat(...targets: Array<AxiosHeaders | RawAxiosHeaders | string>): AxiosHeaders;
+  concat(...targets: Array<AxiosHeaders | RawAxiosHeaders | string | undefined | null>): AxiosHeaders;
 
   toJSON(asStrings?: boolean): RawAxiosHeaders;
 
@@ -47,32 +40,42 @@ export class AxiosHeaders {
 
   static accessor(header: string | string[]): AxiosHeaders;
 
-  static concat(...targets: Array<AxiosHeaders | RawAxiosHeaders | string>): AxiosHeaders;
+  static concat(...targets: Array<AxiosHeaders | RawAxiosHeaders | string | undefined | null>): AxiosHeaders;
 
-  setContentType: AxiosHeaderSetter;
-  getContentType: AxiosHeaderGetter;
-  hasContentType: AxiosHeaderTester;
+  setContentType(value: AxiosHeaderValue, rewrite?: boolean | AxiosHeaderMatcher): AxiosHeaders;
+  getContentType(parser?: RegExp): RegExpExecArray | null;
+  getContentType(matcher?: AxiosHeaderMatcher): AxiosHeaderValue;
+  hasContentType(matcher?: AxiosHeaderMatcher): boolean;
 
-  setContentLength: AxiosHeaderSetter;
-  getContentLength: AxiosHeaderGetter;
-  hasContentLength: AxiosHeaderTester;
+  setContentLength(value: AxiosHeaderValue, rewrite?: boolean | AxiosHeaderMatcher): AxiosHeaders;
+  getContentLength(parser?: RegExp): RegExpExecArray | null;
+  getContentLength(matcher?: AxiosHeaderMatcher): AxiosHeaderValue;
+  hasContentLength(matcher?: AxiosHeaderMatcher): boolean;
 
-  setAccept: AxiosHeaderSetter;
-  getAccept: AxiosHeaderGetter;
-  hasAccept: AxiosHeaderTester;
+  setAccept(value: AxiosHeaderValue, rewrite?: boolean | AxiosHeaderMatcher): AxiosHeaders;
+  getAccept(parser?: RegExp): RegExpExecArray | null;
+  getAccept(matcher?: AxiosHeaderMatcher): AxiosHeaderValue;
+  hasAccept(matcher?: AxiosHeaderMatcher): boolean;
 
-  setUserAgent: AxiosHeaderSetter;
-  getUserAgent: AxiosHeaderGetter;
-  hasUserAgent: AxiosHeaderTester;
+  setUserAgent(value: AxiosHeaderValue, rewrite?: boolean | AxiosHeaderMatcher): AxiosHeaders;
+  getUserAgent(parser?: RegExp): RegExpExecArray | null;
+  getUserAgent(matcher?: AxiosHeaderMatcher): AxiosHeaderValue;
+  hasUserAgent(matcher?: AxiosHeaderMatcher): boolean;
 
-  setContentEncoding: AxiosHeaderSetter;
-  getContentEncoding: AxiosHeaderGetter;
-  hasContentEncoding: AxiosHeaderTester;
+  setContentEncoding(value: AxiosHeaderValue, rewrite?: boolean | AxiosHeaderMatcher): AxiosHeaders;
+  getContentEncoding(parser?: RegExp): RegExpExecArray | null;
+  getContentEncoding(matcher?: AxiosHeaderMatcher): AxiosHeaderValue;
+  hasContentEncoding(matcher?: AxiosHeaderMatcher): boolean;
+
+  setAuthorization(value: AxiosHeaderValue, rewrite?: boolean | AxiosHeaderMatcher): AxiosHeaders;
+  getAuthorization(parser?: RegExp): RegExpExecArray | null;
+  getAuthorization(matcher?: AxiosHeaderMatcher): AxiosHeaderValue;
+  hasAuthorization(matcher?: AxiosHeaderMatcher): boolean;
 
   [Symbol.iterator](): IterableIterator<[string, AxiosHeaderValue]>;
 }
 
-type CommonRequestHeadersList = 'Accept' | 'Content-Type' | 'Content-Length' | 'User-Agent'| 'Content-Encoding';
+type CommonRequestHeadersList = 'Accept' | 'Content-Type' | 'Content-Length' | 'User-Agent' | 'Content-Encoding' | 'Authorization';
 
 export type RawAxiosRequestHeaders = Partial<RawAxiosHeaders & {
   [Key in CommonRequestHeadersList]: AxiosHeaderValue;
diff --git a/test/module/typings/cjs/index.ts b/test/module/typings/cjs/index.ts
index 3a270cfe..fdab6bec 100644
--- a/test/module/typings/cjs/index.ts
+++ b/test/module/typings/cjs/index.ts
@@ -496,3 +496,14 @@ for (const [header, value] of headers) {
       (error: any) => Promise.reject(error)
   );
 }
+
+{
+  const config: axios.AxiosRequestConfig = {headers: new axios.AxiosHeaders({foo: 1})};
+
+  axios.get('', {
+    headers: {
+      bar: 2,
+      ...config.headers
+    }
+  });
+}
diff --git a/test/module/typings/esm/index.ts b/test/module/typings/esm/index.ts
index c2e29da4..d7d4cdd9 100644
--- a/test/module/typings/esm/index.ts
+++ b/test/module/typings/esm/index.ts
@@ -616,3 +616,14 @@ for (const [header, value] of headers) {
       (error: any) => Promise.reject(error)
   );
 }
+
+{
+  const config: AxiosRequestConfig = {headers: new AxiosHeaders({foo: 1})};
+
+  axios.get('', {
+    headers: {
+      bar: 2,
+      ...config.headers
+    }
+  });
+}