diff --git a/app/models/problem.rb b/app/models/problem.rb
index 62f32fa..bdc8f3c 100644
--- a/app/models/problem.rb
+++ b/app/models/problem.rb
@@ -2,6 +2,14 @@
 # reported as various Errs, but the user has grouped the
 # Errs together as belonging to the same problem.
 
+## Add methode nan? in Time because needed by #max(:created_at)
+#
+# Fix on  Mongoid > 2.3.x with commit :
+# https://github.com/mongoid/mongoid/commit/5481556e24480f0a1783f85d6b5b343b0cef7192
+class Time
+  def nan?; false ;end
+end
+
 class Problem
   include Mongoid::Document
   include Mongoid::Timestamps
diff --git a/spec/models/problem_spec.rb b/spec/models/problem_spec.rb
index c39db3f..f78ccb8 100644
--- a/spec/models/problem_spec.rb
+++ b/spec/models/problem_spec.rb
@@ -91,6 +91,11 @@ describe Problem do
       problem = Fabricate(:problem)
       problem.should_not be_resolved
       problem.stub!(:valid?).and_return(false)
+      ## update_attributes not test #valid? but #errors.any?
+      # https://github.com/mongoid/mongoid/blob/master/lib/mongoid/persistence.rb#L137
+      er = ActiveModel::Errors.new(problem)
+      er.add_on_blank(:resolved)
+      problem.stub!(:errors).and_return(er)
       problem.should_not be_valid
       lambda {
         problem.resolve!
--
libgit2 0.21.2