diff --git a/ldclient/client.py b/ldclient/client.py
index 1e757739..3f5c6da6 100644
--- a/ldclient/client.py
+++ b/ldclient/client.py
@@ -196,10 +196,12 @@ def _send(self, event):
             self._queue.put(event)
 
     def track(self, event_name, user, data=None):
+        self._sanitize_user(user)
         self._send({'kind': 'custom', 'key': event_name,
                     'user': user, 'data': data})
 
     def identify(self, user):
+        self._sanitize_user(user)
         self._send({'kind': 'identify', 'key': user['key'], 'user': user})
 
     def set_offline(self):
@@ -223,7 +225,7 @@ def get_flag(self, key, user, default=False):
         return self.toggle(key, user, default)
 
     def toggle(self, key, user, default=False):
-
+        self._sanitize_user(user)
         default = self._config.get_default(key, default)
 
         if self._offline:
@@ -251,4 +253,8 @@ def cb(feature):
                     'Unhandled exception. Returning default value for flag.')
                 return cb(None)
 
+    def _sanitize_user(self, user):
+        if 'key' in user:
+            user['key'] = str(user['key'])
+
 __all__ = ['LDClient', 'Config']
diff --git a/testing/test_ldclient.py b/testing/test_ldclient.py
index 438e63d2..1c8c0b31 100644
--- a/testing/test_ldclient.py
+++ b/testing/test_ldclient.py
@@ -48,6 +48,15 @@ def get(self, key, callback):
     }
 }
 
+numeric_key_user = {}
+
+sanitized_numeric_key_user = {
+    u'key': '33',
+    u'custom': {
+        u'bizzle': u'def'
+    }
+}
+
 
 class MockConsumer(object):
 
@@ -76,6 +85,13 @@ def noop_consumer():
 
 
 def setup_function(function):
+    global numeric_key_user
+    numeric_key_user = {
+        u'key': 33,
+        u'custom': {
+            u'bizzle': u'def'
+        }
+    }
     client.set_online()
     client._queue = queue.Queue(10)
     client._consumer = mock_consumer()
@@ -119,6 +135,18 @@ def expected_event(e):
 
     assert expected_event(client._queue.get(False))
 
+def test_sanitize_user():
+    client._sanitize_user(numeric_key_user)
+    assert numeric_key_user == sanitized_numeric_key_user
+
+def test_toggle_event_numeric_user_key():
+    client.toggle('feature.key', numeric_key_user, default=None)
+
+    def expected_event(e):
+        return e['kind'] == 'feature' and e['key'] == 'feature.key' and e['user'] == sanitized_numeric_key_user and e['value'] == True and e['default'] == None
+
+    assert expected_event(client._queue.get(False))
+
 
 def test_toggle_event_offline():
     client.set_offline()
@@ -135,6 +163,15 @@ def expected_event(e):
     assert expected_event(client._queue.get(False))
 
 
+def test_identify_numeric_key_user():
+    client.identify(numeric_key_user)
+
+    def expected_event(e):
+        return e['kind'] == 'identify' and e['key'] == '33' and e['user'] == sanitized_numeric_key_user
+
+    assert expected_event(client._queue.get(False))
+
+
 def test_identify_offline():
     client.set_offline()
     client.identify(user)
@@ -150,6 +187,15 @@ def expected_event(e):
     assert expected_event(client._queue.get(False))
 
 
+def test_track_numeric_key_user():
+    client.track('my_event', numeric_key_user, 42)
+
+    def expected_event(e):
+        return e['kind'] == 'custom' and e['key'] == 'my_event' and e['user'] == sanitized_numeric_key_user and e['data'] == 42
+
+    assert expected_event(client._queue.get(False))
+
+
 def test_track_offline():
     client.set_offline()
     client.track('my_event', user, 42)