diff --git a/src/index.js b/src/index.js
index 373e5f2..41cbd4a 100644
--- a/src/index.js
+++ b/src/index.js
@@ -1,19 +1,24 @@
 const crypto = require('crypto');
 const bip39 = require('bip39');
 
-const PREFIX = [0xC2, 0xFE, 0x58];
+const PREFIX = Buffer.from('C2FE58', 'hex');
 
 const P2PKH = 0x00;
 const P2WPKHP2SH = 0x01;
 const P2WPKH = 0x02;
 
+const getMnemonicBuffer = mnemonic => Buffer.from(
+	bip39.mnemonicToEntropy(mnemonic),
+	'hex'
+);
+
 const bip39v = {
 	versions: {P2PKH, P2WPKHP2SH, P2WPKH}
 };
 
 bip39v.generateMnemonic = (length = 224, versionByte = P2PKH) => {
 	const buffer = Buffer.concat([
-		Buffer.from(PREFIX),
+		PREFIX,
 		Buffer.from([versionByte]),
 		crypto.randomBytes(length / 8)
 	]);
@@ -21,11 +26,18 @@ bip39v.generateMnemonic = (length = 224, versionByte = P2PKH) => {
 	return bip39.entropyToMnemonic(buffer);
 };
 
+bip39v.isVersionedMnemonic = mnemonic => {
+	const buffer = getMnemonicBuffer(mnemonic);
+
+	return buffer.slice(0, PREFIX.length).equals(PREFIX);
+};
+
 bip39v.mnemonicToVersionByte = mnemonic => {
-	const buffer = Buffer.from(
-		bip39.mnemonicToEntropy(mnemonic),
-		'hex'
-	);
+	if (!bip39v.isVersionedMnemonic(mnemonic)) {
+		throw new Error('Cannot get version byte of non-versioned mnemonic.');
+	}
+
+	const buffer = getMnemonicBuffer(mnemonic);
 
 	return buffer[PREFIX.length];
 };
diff --git a/test/unit.js b/test/unit.js
index b2de20b..46d49af 100644
--- a/test/unit.js
+++ b/test/unit.js
@@ -11,8 +11,9 @@ const fixtures = {
 		P2PKH: 'seed version abandon bus rebuild logic connect wise illegal traffic transfer olympic royal style equal',
 		P2WPKHP2SH: 'seed version about math twice crater force critic grace panic party label flag draft sketch',
 		P2WPKH: 'seed version absent reward pipe sketch clarify sight spread addict divorce idle burst alarm tide',
-		UNKNOWN: 'such galaxy much glimpse music turkey toward exhaust filter key pilot hello'
-	}
+		UNKNOWN: 'seed version divide rural field error snack clump gather shift globe mask trend october ten'
+	},
+	nonVersionedMnemonic: 'much bottom such hurt hunt welcome cushion erosion pulse admit name deer'
 };
 
 test('bip39v is exported', t => {
@@ -48,10 +49,19 @@ test('bip39v.generateMnemonic(length, version) adds correct version', t => {
 	});
 });
 
+test('bip39v.isVersionedMnemonic(mnemonic) against fixture data', t => {
+	Object.values(fixtures.mnemonics).forEach(mnemonic => {
+		const isVersionedMnemonic = bip39v.isVersionedMnemonic(mnemonic);
+		t.true(isVersionedMnemonic);
+	});
+	const isVersionedMnemonic = bip39v.isVersionedMnemonic(fixtures.nonVersionedMnemonic);
+	t.false(isVersionedMnemonic);
+});
+
 test('bip39v.mnemonicToVersionByte(mnemonic) against fixture data', t => {
 	Object.entries(fixtures.mnemonics).forEach(([version, mnemonic]) => {
 		const versionByte = bip39v.mnemonicToVersionByte(mnemonic);
-		const expectedVersionByte = version === 'UNKNOWN' ? 0x44 : fixtures.versionBytes[version];
+		const expectedVersionByte = version === 'UNKNOWN' ? 0xFF : fixtures.versionBytes[version];
 		t.is(versionByte, expectedVersionByte);
 	});
 });