diff --git a/packages/portal/package.json b/packages/portal/package.json
index d05e79a..9b1426c 100644
--- a/packages/portal/package.json
+++ b/packages/portal/package.json
@@ -15,7 +15,8 @@
 		"build:umd:gzip":
 			"npm run build:umd:min && gzip -c9 dist/worker-portal.min.js > dist/worker-portal.min.js.gz",
 		"build": "npm run build:umd:gzip && ls -l dist/",
-		"prepublish": "npm run clean && npm run build"
+		"prepublish": "npm run clean && npm run build",
+		"test": "npm run build:lib && ava --verbose"
 	},
 	"repository": {
 		"type": "git",
@@ -29,6 +30,7 @@
 	},
 	"homepage": "https://gitlab.com/explorigin/worker-portal",
 	"devDependencies": {
+		"ava": "^0.17.0",
 		"babel-cli": "6.18.0",
 		"babel-core": "6.21.0",
 		"babel-eslint": "7.1.1",
diff --git a/packages/portal/src/index.js b/packages/portal/src/index.js
index f787777..ac61314 100644
--- a/packages/portal/src/index.js
+++ b/packages/portal/src/index.js
@@ -12,14 +12,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-export function isWorker() {
-	try {
-		return self !== window;
-	} catch (e) {
-		return true;
-	}
-}
-
 function tryJSON(type, destination, id, data) {
 	const packet = [type, destination, id, data];
 	try {
@@ -29,7 +21,7 @@ function tryJSON(type, destination, id, data) {
 	}
 }
 
-export function WorkerPortal(context, worker, serialize) {
+export function WorkerPortal(context, worker, isSlave, serialize) {
 	const responseMap = new Map();
 	const _worker = worker || self;
 	const contextIndex = Object.keys(context);
@@ -84,29 +76,25 @@ export function WorkerPortal(context, worker, serialize) {
 	}
 
 	function injectionPointFactory(fnId, callbackFactory) {
-		return () =>
-			new Promise((resolve, reject) => {
-				if (!enabled) {
+		return function(...args) {
+			return new Promise((resolve, reject) => {
+				if (!enabled && fnId !== 0) {
 					reject(new Error('Portal disabled'));
 				}
 				const id = callCount;
 				callCount += 1;
 				responseMap.set(id, [callbackFactory ? callbackFactory(resolve) : resolve, reject]);
-				post(0, id, fnId, Array.from(arguments));
+				post(0, id, fnId, args);
 			});
+		};
 	}
 
 	function resolveExternalInterfaceFactory(resolve) {
-		return linkedFunctionNames => {
+		return (linkedFunctionNames, ...rest) => {
 			const externalInterface = {};
 			linkedFunctionNames.forEach((fnName, index) => {
 				externalInterface[fnName] = injectionPointFactory(index);
 			});
-			if (!isWorker()) {
-				externalInterface._cleanup = injectionPointFactory(linkedFunctionNames.length, resolve =>
-					resolve(cleanup())
-				);
-			}
 			enabled = true;
 			resolve(externalInterface);
 			return contextIndex;
@@ -125,13 +113,16 @@ export function WorkerPortal(context, worker, serialize) {
 
 	_worker.addEventListener('message', dispatcher);
 
-	if (isWorker()) {
+	if (isSlave) {
 		return new Promise(resolve => {
-			contextIndex.splice(0, 0, '__init', '__cleanup');
+			contextIndex.splice(0, 0, '__init', '__cleanupSlave');
 			context.__init = resolveExternalInterfaceFactory(resolve);
-			context.__cleanup = cleanup;
+			context.__cleanupSlave = cleanup;
 		});
 	}
 
-	return injectionPointFactory(0, resolveExternalInterfaceFactory)(contextIndex);
+	return injectionPointFactory(0, resolveExternalInterfaceFactory)(contextIndex).then(api => ({
+		...api,
+		_cleanup: injectionPointFactory(1, resolve => resolve(cleanup()))
+	}));
 }
diff --git a/packages/portal/test.js b/packages/portal/test.js
new file mode 100644
index 0000000..c0e23e0
--- /dev/null
+++ b/packages/portal/test.js
@@ -0,0 +1,80 @@
+import test from 'ava';
+
+import { WorkerPortal } from './lib';
+
+function FakeWorkerPair() {
+	let cbA = null;
+	let cbB = null;
+
+	const objA = {
+		postMessage: data => {
+			cbB({ data: data });
+		},
+		addEventListener: (eventName, fn) => {
+			cbA = (...r) => {
+				fn(...r);
+			};
+		},
+		removeEventListener: (eventName, fn) => {
+			cbA = null;
+		}
+	};
+
+	const objB = {
+		postMessage: data => {
+			cbA({ data: data });
+		},
+		addEventListener: (eventName, fn) => {
+			cbB = (...r) => {
+				fn(...r);
+			};
+		},
+		removeEventListener: (eventName, fn) => {
+			cbB = null;
+		}
+	};
+
+	return [objA, objB];
+}
+
+test('Workers can call and respond equally', async t => {
+	const [a, b] = FakeWorkerPair();
+
+	const slave = WorkerPortal(
+		{
+			slaveAdd: (a, b) => a + b
+		},
+		a,
+		true
+	);
+	const master = WorkerPortal(
+		{
+			masterSubtract: (a, b) => a - b
+		},
+		b,
+		false
+	);
+
+	const masterApi = await master;
+	const slaveApi = await slave;
+
+	t.deepEqual(Object.keys(masterApi), ['__init', '__cleanupSlave', 'slaveAdd', '_cleanup']);
+	t.deepEqual(Object.keys(slaveApi), ['masterSubtract']);
+
+	t.is(await slaveApi.masterSubtract(9, 2), 7);
+	t.is(await masterApi.slaveAdd(9, 2), 11);
+
+	t.is(await masterApi.slaveAdd(5, 2), 7);
+	t.is(await slaveApi.masterSubtract(2, 2), 0);
+
+	await masterApi._cleanup();
+
+	return masterApi
+		.slaveAdd(9, 2)
+		.then(e => {
+			t.fail('Expected rejection');
+		})
+		.catch(e => {
+			t.is(e.message, 'Portal disabled');
+		});
+});