Response serialization fails for date pick one answers

Given I start the survey below and..
1. Select "Date" without entering a date
2. Serialize the response to JSON, 
3. I will see an exception "undefined method `strftime' for nil:NilClass".

<pre>
survey "Health" do
  section "Doctor" do
    question "When did you visit?", :pick => :one
    a "Date", :date
    a "Not sure"
  end
end
</pre>