diff --git a/package.json b/package.json
index 88f06f0..d590e2a 100644
--- a/package.json
+++ b/package.json
@@ -49,8 +49,8 @@
     "eslint-plugin-prettier": "^4.2.1",
     "mocha": "^10.0.0",
     "prettier": "^2.8.7",
-    "ts-node": "^10.9.1",
     "ts-loader": "^9.4.4",
+    "ts-node": "^10.9.1",
     "typescript": "^5.0.3"
   }
 }
diff --git a/src/sign.ts b/src/sign.ts
index 554a5d5..fbb45d5 100644
--- a/src/sign.ts
+++ b/src/sign.ts
@@ -43,10 +43,8 @@ export async function sign<T extends RequestLike | ResponseLike>(message: T, opt
     message.headers.set('Signature', `${key}=:${sigBase64}:`);
     message.headers.set('Signature-Input', `${key}=${signatureInputString}`);
   } else {
-    Object.assign(message.headers, {
-      Signature: `${key}=:${sigBase64}:`,
-      'Signature-Input': `${key}=${signatureInputString}`,
-    });
+    message.headers['Signature'] = `${key}=:${sigBase64}:`;
+    message.headers['Signature-Input'] = `${key}=${signatureInputString}`;
   }
 
   return message;
diff --git a/src/types.ts b/src/types.ts
index aa7d8c1..d6c9f0a 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -22,7 +22,7 @@ interface HeadersMap {
   set(name: string, value: string): void;
 }
 
-type Headers = Record<string, string> | HeadersMap;
+type Headers = Record<string, HeaderValue> | HeadersMap;
 
 export type HeaderValue = { toString(): string } | string | string[] | undefined;