diff --git a/src/lease.ts b/src/lease.ts
index 313c1a4..be1e76e 100644
--- a/src/lease.ts
+++ b/src/lease.ts
@@ -70,7 +70,7 @@ const enum State {
  * const hostPrefix = 'available-hosts/';
  *
  * function grantLease() {
- *   const lease = client.lease();
+ *   const lease = client.lease(10); // set a TTL of 10 seconds
  *
  *   lease.on('lost', err => {
  *     console.log('We lost our lease as a result of this error:', err);
@@ -102,7 +102,7 @@ export class Lease extends EventEmitter {
   ) {
     super();
 
-    if (ttl < 1) {
+    if (!ttl || ttl < 1) {
       throw new Error(`The TTL in an etcd lease must be at least 1 second. Got: ${ttl}`);
     }
 
diff --git a/test/lease.test.ts b/test/lease.test.ts
index e7400c5..506821c 100644
--- a/test/lease.test.ts
+++ b/test/lease.test.ts
@@ -28,8 +28,9 @@ describe('lease()', () => {
     }
   });
 
-  it('throws if trying to use too short of a ttl', () => {
+  it('throws if trying to use too short of a ttl, or an undefined ttl', () => {
     expect(() => client.lease(0)).to.throw(/must be at least 1 second/);
+    expect(() => (<any> client.lease)()).to.throw(/must be at least 1 second/);
   });
 
   it('reports a loss and errors if the client is invalid', async () => {