diff --git a/archive/_common.ts b/archive/_common.ts
index f383bb9a03e6..0f6c427b093b 100644
--- a/archive/_common.ts
+++ b/archive/_common.ts
@@ -71,7 +71,7 @@ struct posix_header {           // byte offset
 };
 */
 
-export const ustarStructure: Array<{ field: string; length: number }> = [
+export const ustarStructure = [
   {
     field: "fileName",
     length: 100,
@@ -136,7 +136,9 @@ export const ustarStructure: Array<{ field: string; length: number }> = [
     field: "padding",
     length: 12,
   },
-];
+] as const;
+
+export type UstarFields = (typeof ustarStructure)[number]["field"];
 
 export async function readBlock(
   reader: Reader,
diff --git a/archive/untar.ts b/archive/untar.ts
index bf9feec71a02..0a597f9cd2dc 100644
--- a/archive/untar.ts
+++ b/archive/untar.ts
@@ -34,6 +34,7 @@ import {
   HEADER_LENGTH,
   readBlock,
   type TarMeta,
+  UstarFields,
   ustarStructure,
 } from "./_common.ts";
 import { readAll } from "../streams/read_all.ts";
@@ -47,9 +48,9 @@ export interface TarMetaWithLinkName extends TarMeta {
   linkName?: string;
 }
 
-export interface TarHeader {
-  [key: string]: Uint8Array;
-}
+export type TarHeader = {
+  [key in UstarFields]: Uint8Array;
+};
 
 // https://pubs.opengroup.org/onlinepubs/9699919799/utilities/pax.html#tag_20_92_13_06
 // eight checksum bytes taken to be ascii spaces (decimal value 32)
@@ -69,8 +70,8 @@ function trim(buffer: Uint8Array): Uint8Array {
  * Parse file header in a tar archive
  * @param length
  */
-function parseHeader(buffer: Uint8Array): { [key: string]: Uint8Array } {
-  const data: { [key: string]: Uint8Array } = {};
+function parseHeader(buffer: Uint8Array): TarHeader {
+  const data = {} as TarHeader;
   let offset = 0;
   ustarStructure.forEach(function (value) {
     const arr = buffer.subarray(offset, offset + value.length);
@@ -222,7 +223,7 @@ export class Untar {
         // Ignore checksum header
         continue;
       }
-      sum += header[i];
+      sum += header[i]!;
     }
     return sum;
   }
diff --git a/archive/untar_test.ts b/archive/untar_test.ts
index 1f3405dd1010..9f66e809223c 100644
--- a/archive/untar_test.ts
+++ b/archive/untar_test.ts
@@ -332,7 +332,22 @@ Deno.test({
     // Test TarEntry type
     const bufSizes = [1, 53, 256, 511];
     const header: TarHeader = {
-      test: new Uint8Array(bufSizes),
+      fileName: new Uint8Array(bufSizes),
+      fileMode: new Uint8Array(bufSizes),
+      uid: new Uint8Array(bufSizes),
+      gid: new Uint8Array(bufSizes),
+      fileSize: new Uint8Array(bufSizes),
+      mtime: new Uint8Array(bufSizes),
+      checksum: new Uint8Array(bufSizes),
+      type: new Uint8Array(bufSizes),
+      linkName: new Uint8Array(bufSizes),
+      ustar: new Uint8Array(bufSizes),
+      owner: new Uint8Array(bufSizes),
+      group: new Uint8Array(bufSizes),
+      majorNumber: new Uint8Array(bufSizes),
+      minorNumber: new Uint8Array(bufSizes),
+      fileNamePrefix: new Uint8Array(bufSizes),
+      padding: new Uint8Array(bufSizes),
     };
     const content = new TextEncoder().encode("hello tar world!");
     const reader = new Buffer(content);