Check prefix is set

src/index.js

@@ -13,6 +13,10 @@ class Vain extends Emitter {
 		super();
 		this.addressFormat = addressFormats.get(addressFormat);
 
+		if (typeof prefix !== 'string' || prefix.length === 0) {
+			throw new Error('Prefix must be set');
+		}
+
 		prefix.split('').forEach(char => {
 			if (!this.addressFormat.charset.includes(char)) {
 				throw new Error(`Invalid characters for address format "${addressFormat}"`);

test/unit.js

@@ -88,3 +88,9 @@ test('Vain throws on invalid charset', t => {
 		t.true(error.message.includes('Invalid characters for address format'));
 	});
 });
+
+test('Vain throws on undefined prefix', t => {
+	const options = {};
+	const error = t.throws(() => new Vain(options));
+	t.is(error.message, 'Prefix must be set');
+});