diff --git a/app/models/repository.rb b/app/models/repository.rb
index b0bf0d2a7..59110fd4d 100644
--- a/app/models/repository.rb
+++ b/app/models/repository.rb
@@ -119,7 +119,7 @@ class Repository < ActiveRecord::Base
     elsif repository.is_default?
       1
     else
-      identifier <=> repository.identifier
+      identifier.to_s <=> repository.identifier.to_s
     end
   end
 
diff --git a/test/unit/repository_test.rb b/test/unit/repository_test.rb
index 1fe246d8e..466babb38 100644
--- a/test/unit/repository_test.rb
+++ b/test/unit/repository_test.rb
@@ -318,4 +318,13 @@ class RepositoryTest < ActiveSupport::TestCase
     assert_equal "test_value_23",
                  repo.extra_info["test_2"]["test_23"]
   end
+
+  def test_sort_should_not_raise_an_error_with_nil_identifiers
+    r1 = Repository.new
+    r2 = Repository.new
+
+    assert_nothing_raised do
+      [r1, r2].sort
+    end
+  end
 end