diff --git a/googleapiclient/discovery.py b/googleapiclient/discovery.py
index c798115..7244c5b 100644
--- a/googleapiclient/discovery.py
+++ b/googleapiclient/discovery.py
@@ -29,7 +29,7 @@
 
 # Standard library imports
 import copy
-
+from collections import OrderedDict
 try:
     from email.generator import BytesGenerator
 except ImportError:
@@ -241,7 +241,8 @@
     else:
         discovery_http = http
 
-    for discovery_url in (discoveryServiceUrl, V2_DISCOVERY_URI):
+    for discovery_url in \
+            _discovery_service_uri_options(discoveryServiceUrl, version):
         requested_url = uritemplate.expand(discovery_url, params)
 
         try:
@@ -270,6 +271,29 @@
     raise UnknownApiNameOrVersion("name: %s  version: %s" % (serviceName, version))
 
 
+def _discovery_service_uri_options(discoveryServiceUrl, version):
+    """
+    Returns Discovery URIs to be used for attemnting to build the API Resource.
+
+  Args:
+    discoveryServiceUrl:
+        string, the Original Discovery Service URL preferred by the customer.
+    version:
+        string, API Version requested
+
+  Returns:
+      A list of URIs to be tried for the Service Discovery, in order.
+    """
+
+    urls = [discoveryServiceUrl, V2_DISCOVERY_URI]
+    # V1 Discovery won't work if the requested version is None
+    if discoveryServiceUrl == V1_DISCOVERY_URI and version is None:
+        logger.warning(
+            "Discovery V1 does not support empty versions. Defaulting to V2...")
+        urls.pop(0)
+    return list(OrderedDict.fromkeys(urls))
+
+
 def _retrieve_discovery_doc(url, http, cache_discovery,
     cache=None, developerKey=None, num_retries=1):
     """Retrieves the discovery_doc from cache or the internet.
diff --git a/tests/test_discovery.py b/tests/test_discovery.py
index f59ea15..87cc8ed 100644
--- a/tests/test_discovery.py
+++ b/tests/test_discovery.py
@@ -837,6 +837,37 @@
         )
         self.assertEqual(zoo._baseUrl, api_endpoint)
 
+    def test_discovery_with_empty_version_uses_v2(self):
+        http = HttpMockSequence(
+            [
+                ({"status": "200"}, read_datafile("zoo.json", "rb")),
+            ]
+        )
+        build("zoo", version=None, http=http, cache_discovery=False)
+        validate_discovery_requests(self, http, "zoo", None, V2_DISCOVERY_URI)
+
+    def test_discovery_with_empty_version_preserves_custom_uri(self):
+        http = HttpMockSequence(
+            [
+                ({"status": "200"}, read_datafile("zoo.json", "rb")),
+            ]
+        )
+        custom_discovery_uri = "https://foo.bar/$discovery"
+        build(
+            "zoo", version=None, http=http,
+            cache_discovery=False, discoveryServiceUrl=custom_discovery_uri)
+        validate_discovery_requests(
+            self, http, "zoo", None, custom_discovery_uri)
+
+    def test_discovery_with_valid_version_uses_v1(self):
+        http = HttpMockSequence(
+            [
+                ({"status": "200"}, read_datafile("zoo.json", "rb")),
+            ]
+        )
+        build("zoo", version="v123", http=http, cache_discovery=False)
+        validate_discovery_requests(self, http, "zoo", "v123", V1_DISCOVERY_URI)
+
 
 class DiscoveryRetryFromHttp(unittest.TestCase):
     def test_repeated_500_retries_and_fails(self):