Excel- Data Validation Using Apache Poi

Hi All,

I'm trying to validate data using an excel sheet which contains data validations. e.g. I've created list data validation on column named category which allows values (X, Y, Z and " "). But I've observed that because of the blank value in the source list, it allow any invalid values ( A,B). My intention to add a blank value in the source list is to allow a blank value only. But it violates my purpose. 
Is it the correct behaviour or should it only allow values which are there in the source list (including blank value)?

Thanks