Set custom filter values as 'blank' or null

Hi,

How to set value as blank to custom filter.

Following code which will create custom filter for blank value as null but it throws following error 

"values.0 is invalid. Not meeting type constraint"

code

custom visual code

var customFilter={
$schema: "http://powerbi.com/product/schema#basic",
target: {
table: filters[1].target.table,
column: filters[1].target.column
},
operator: filters[1].operator ,
values:[null]
}

So which value pass in custom filter so it will display as (Blank) in filter panel?

Thanks in Advance.