I am encountering an error when I run a simple query on the Contact record. Email is again a Standard Field 

SELECT Id FROM Contact WHERE Email = 'asd@gmail.com'

I get the below error:-
"
[object Object]: SELECT Id FROM Contact WHERE Email = 'asd@gmail.com' ^ ERROR at Row:1:Column:30 field 'Email' can not be filtered in a query call
"


Why can't I filter using a Standard (non text area field)??