diff --git a/js/index.ts b/js/index.ts
index 4d8bc03..7f8ffa9 100644
--- a/js/index.ts
+++ b/js/index.ts
@@ -309,8 +309,15 @@ export class FetchSource implements Source {
 
 export function bytesToHeader(bytes: ArrayBuffer, etag?: string): Header {
 	const v = new DataView(bytes);
+	const spec_version = v.getUint8(7);
+	if (spec_version > 3) {
+		throw Error(
+			`Archive is spec version ${spec_version} but this library supports up to spec version 3`
+		);
+	}
+
 	return {
-		specVersion: 3,
+		specVersion: spec_version,
 		rootDirectoryOffset: Number(v.getBigUint64(8, true)),
 		rootDirectoryLength: Number(v.getBigUint64(16, true)),
 		jsonMetadataOffset: Number(v.getBigUint64(24, true)),
diff --git a/js/test/data/invalid_v4.pmtiles b/js/test/data/invalid_v4.pmtiles
new file mode 100644
index 0000000..1871cb2
Binary files /dev/null and b/js/test/data/invalid_v4.pmtiles differ
diff --git a/js/test/v3.test.ts b/js/test/v3.test.ts
index bec25b7..4238a95 100644
--- a/js/test/v3.test.ts
+++ b/js/test/v3.test.ts
@@ -192,7 +192,17 @@ test("cache check magic number", async (assertion) => {
 		await cache.getHeader(source);
 		assertion.fail("Should have thrown");
 	} catch (e) {
-		console.log(e);
+		assertion.ok(e instanceof Error);
+	}
+});
+
+test("cache check future spec version", async (assertion) => {
+	const source = new TestNodeFileSource("test/data/invalid_v4.pmtiles", "1");
+	const cache = new SharedPromiseCache();
+	try {
+		await cache.getHeader(source);
+		assertion.fail("Should have thrown");
+	} catch (e) {
 		assertion.ok(e instanceof Error);
 	}
 });