Empty submit can add a error POST,how can do?

when I test rforum, i do a /topic/new .
I have not type anything but submit empty ‘Subject’ and empty ‘Text’.
Then return information :“Please enter a longer subject” …“Please
enter a longer text.”.

now, do /forum/1 then get:

TypeError in Forum#forum
Showing app/views/forum/forum.rhtml where line #22 raised:

no implicit conversion to float from nil

Extracted source (around line #22):

20: <%=h (topic.post_counter - 1) %>
22: <%=h format_relative_time(topic.last_post_created_at) %>
23: <%=l :by %> <%=h topic.last_post_author %>

I found there has added a topic in datebase
this topic have null last_post_created_at last_post_user_id
last_post_author .

please help me.How can I do?