diff --git a/app/models/project.rb b/app/models/project.rb
index e636c33..cdb460e 100644
--- a/app/models/project.rb
+++ b/app/models/project.rb
@@ -253,7 +253,7 @@ class Project < ActiveRecord::Base
 
   def to_param
     # id is used for projects with a numeric identifier (compatibility)
-    @to_param ||= (identifier.to_s =~ %r{^\d*$} ? id : identifier)
+    @to_param ||= (identifier.to_s =~ %r{^\d*$} ? id.to_s : identifier)
   end
 
   def active?
diff --git a/test/unit/helpers/application_helper_test.rb b/test/unit/helpers/application_helper_test.rb
index c5d644c..b261fd3 100644
--- a/test/unit/helpers/application_helper_test.rb
+++ b/test/unit/helpers/application_helper_test.rb
@@ -925,6 +925,14 @@ RAW
                  link_to_project(project, {:action => 'settings'}, :class => "project")
   end
 
+  def test_link_to_legacy_project_with_numerical_identifier_should_use_id
+    # numeric identifier are no longer allowed
+    Project.update_all "identifier=25", "id=1"
+
+    assert_equal '<a href="/projects/1">eCookbook</a>',
+                 link_to_project(Project.find(1))
+  end
+
   def test_principals_options_for_select_with_users
     users = [User.find(2), User.find(4)]
     assert_equal %(<option value="2">John Smith</option><option value="4">Robert Hill</option>),