Filtering a variable does not work properly in Kibana4

I have the following column (post_page_event) with several entries. I want to be able to filter on that column to only pick out entries with post_page_event: 0. However, I get a null result. This only happens on select variables. For other variables everything works as expected. Is this a known issue? I includes the screen shots below.

what is the mapping for that field, a number of a string?