diff --git a/lib/mongo/connection.rb b/lib/mongo/connection.rb
index 41bcade..26dcd7c 100644
--- a/lib/mongo/connection.rb
+++ b/lib/mongo/connection.rb
@@ -522,11 +522,11 @@ module Mongo
         port  = port.to_i
         db    = matches[8]
 
-        if (uname || pwd || db) && !(uname && pwd && db)
+        if uname && pwd && db
+          add_auth(db, uname, pwd)
+        elsif uname || pwd || db
           raise MongoArgumentError, "MongoDB URI must include all three of username, password, " +
             "and db if any one of these is specified."
-        else
-          add_auth(db, uname, pwd)
         end
 
         nodes << [host, port]
diff --git a/test/connection_test.rb b/test/connection_test.rb
index 3816616..1dbb790 100644
--- a/test/connection_test.rb
+++ b/test/connection_test.rb
@@ -24,6 +24,12 @@ class TestConnection < Test::Unit::TestCase
     assert Mongo::Support.ok?(server_info)
   end
 
+  def test_connection_uri
+    con = Connection.from_uri("mongodb://localhost:27017")
+    assert_equal "localhost", con.host
+    assert_equal 27017, con.port
+  end
+
   def test_server_version
     assert_match /\d\.\d+(\.\d+)?/, @mongo.server_version.to_s
   end