diff --git a/aiohttp/client_reqrep.py b/aiohttp/client_reqrep.py
index fe723cec6c8..b053f39aafa 100644
--- a/aiohttp/client_reqrep.py
+++ b/aiohttp/client_reqrep.py
@@ -112,6 +112,9 @@ def update_host(self, url):
 
         # get host/port
         host = url_parsed.hostname
+        if not host:
+            raise ValueError('Host could not be detected.')
+
         try:
             port = url_parsed.port
         except ValueError:
diff --git a/tests/test_client_request.py b/tests/test_client_request.py
index 912d9563647..155b23a5e3b 100644
--- a/tests/test_client_request.py
+++ b/tests/test_client_request.py
@@ -96,6 +96,11 @@ def test_host_port_err(self):
             ValueError, ClientRequest, 'get', 'http://python.org:123e/',
             loop=self.loop)
 
+    def test_hostname_err(self):
+        self.assertRaises(
+            ValueError, ClientRequest, 'get', 'http://:8080/',
+            loop=self.loop)
+
     def test_host_header(self):
         req = ClientRequest('get', 'http://python.org/', loop=self.loop)
         self.assertEqual(req.headers['HOST'], 'python.org')