diff --git a/src/__test__/index.test.js b/src/__test__/index.test.js
new file mode 100644
index 0000000..4884266
--- /dev/null
+++ b/src/__test__/index.test.js
@@ -0,0 +1,16 @@
+const DecryptorV4 = require("../crypto/DecryptorV4");
+const jooxFactory = require("../index");
+
+describe("package entry", () => {
+  it("should return null on unsupported file", () => {
+    const input = Buffer.from("E!99........", "utf-8");
+    const decryptor = jooxFactory(input, "".padStart(32, "0"));
+    expect(decryptor).toBeNull();
+  });
+
+  it("should return an encryptor on v4 magic file", () => {
+    const input = Buffer.from("E!04........", "utf-8");
+    const decryptor = jooxFactory(input, "".padStart(32, "0"));
+    expect(decryptor).toBeInstanceOf(DecryptorV4);
+  });
+});
diff --git a/src/index.js b/src/index.js
index 33eeb8c..dabb0a1 100644
--- a/src/index.js
+++ b/src/index.js
@@ -5,7 +5,7 @@ function jooxFactory(fileBody, seed) {
     return new DecryptorV4(seed);
   }
 
-  throw new Error("input file not supported or invalid");
+  return null;
 }
 
 module.exports = jooxFactory;