Custom field not working on search list

I created a new custom field in invoices module called work type which is a select list with fixed 5 values. I am able to enter the data into the same but when you view the invoices in LIST mode, the search field at the top against this new field shows as string not a drop down. Also when i type any word that is in the value, it does not filter the list based on the word.

can anyone help me ??

Hi :happy: