diff --git a/lib/octoherder/configuration.rb b/lib/octoherder/configuration.rb
index b861ccb..9085548 100644
--- a/lib/octoherder/configuration.rb
+++ b/lib/octoherder/configuration.rb
@@ -80,17 +80,24 @@ def update_milestones octokit_connection
       ([master] + repositories).map { |str|
         Octokit::Repository.new str
       }.each { |repo|
-        actual_milestones = octokit_connection.list_milestones(repo).map { |m|
-          m.fetch('title', '')
-        }
-        milestones.reject { |m| actual_milestones.include? m['title'] }.each { |m|
+        ms = octokit_connection.list_milestones(repo)
+
+        # Map milestone titles to IDs
+        actual_milestones = Hash[ms.map { |m|
+          [m.fetch('title'), m.fetch('number')]
+        }]
+
+        milestone_titles = actual_milestones.keys
+
+        milestones.reject { |m| milestone_titles.include? m.fetch('title') }.each { |m|
           opts = to_octokit_opts m
-          octokit_connection.create_milestone(repo, m['title'], opts)
+          octokit_connection.create_milestone(repo, m.fetch('title'), opts)
         }
 
-        milestones.select { |m| actual_milestones.include? m['title'] }.each { |m|
+        milestones.select { |m| milestone_titles.include? m.fetch('title') }.each { |m|
+          milestone_number = actual_milestones[m.fetch('title')]
           opts = to_octokit_opts m
-          octokit_connection.update_milestone(repo, m['title'], opts)
+          octokit_connection.update_milestone(repo, milestone_number, opts)
         }
       }
       self
diff --git a/spec/configuration_spec.rb b/spec/configuration_spec.rb
index df8357f..bda3584 100644
--- a/spec/configuration_spec.rb
+++ b/spec/configuration_spec.rb
@@ -89,13 +89,14 @@ module OctoHerder
       end
 
       it "should update existing milestones" do
-        connection.stub(:list_milestones).and_return([{'title' => 'milestone-1'},
-                                                      {'title' => 'milestone-2'},
-                                                      {'title' => 'milestone-3'}])
+        connection.stub(:list_milestones).and_return([{'number' => 1, 'title' => 'milestone-1'},
+                                                      {'number' => 2, 'title' => 'milestone-2'},
+                                                      {'number' => 3, 'title' => 'milestone-3'}])
         connection.stub(:update_milestone)
-        connection.should_receive(:update_milestone).with(an_instance_of(Octokit::Repository), 'milestone-1', {'state' => 'closed'}).exactly(repo_count).times
-        connection.should_receive(:update_milestone).with(an_instance_of(Octokit::Repository), 'milestone-2', {'due_on' => '2011-04-10T20:09:31Z'}).exactly(repo_count).times
-        connection.should_receive(:update_milestone).with(an_instance_of(Octokit::Repository), 'milestone-3', {'state' => 'open', 'description' => 'The third step in total world domination.'}).exactly(repo_count).times
+
+        connection.should_receive(:update_milestone).with(an_instance_of(Octokit::Repository), 1, {'state' => 'closed'}).exactly(repo_count).times
+        connection.should_receive(:update_milestone).with(an_instance_of(Octokit::Repository), 2, {'due_on' => '2011-04-10T20:09:31Z'}).exactly(repo_count).times
+        connection.should_receive(:update_milestone).with(an_instance_of(Octokit::Repository), 3, {'state' => 'open', 'description' => 'The third step in total world domination.'}).exactly(repo_count).times
 
         conf.update_milestones connection
       end