diff --git a/rq/worker.py b/rq/worker.py
index 7602c2e..c782fd6 100644
--- a/rq/worker.py
+++ b/rq/worker.py
@@ -540,7 +540,10 @@ class Worker(object):
         queues = as_text(queues)
         self._state = as_text(state or '?')
         self._job_id = job_id or None
-        self.last_heartbeat = utcparse(as_text(last_heartbeat))
+        if last_heartbeat:
+            self.last_heartbeat = utcparse(as_text(last_heartbeat))
+        else:
+            self.last_heartbeat = None
         self.birth_date = utcparse(as_text(birth))
         if failed_job_count:
             self.failed_job_count = int(as_text(failed_job_count))
diff --git a/tests/test_worker.py b/tests/test_worker.py
index 3f76e4d..565a899 100644
--- a/tests/test_worker.py
+++ b/tests/test_worker.py
@@ -202,12 +202,16 @@ class TestWorker(RQTestCase):
         q = Queue()
         w = Worker([q])
         w.register_birth()
-        w.heartbeat()
         last_heartbeat = self.testconn.hget(w.key, 'last_heartbeat')
 
         self.assertTrue(last_heartbeat is not None)
         w = Worker.find_by_key(w.key)
-        self.assertIsInstance(w.last_heartbeat, datetime)        
+        self.assertIsInstance(w.last_heartbeat, datetime)   
+
+        # worker.refresh() shouldn't fail if last_heartbeat is None
+        # for compatibility reasons
+        self.testconn.hdel(w.key, 'last_heartbeat')        
+        w.refresh()     
 
     def test_work_fails(self):
         """Failing jobs are put on the failed queue."""