add is null condtion in db_update I want to use db_update with condition that a specify field is null ,how can I do it?

something like 

-&gt;condition('used_date',  'isnull','=') 


is this correct ,if not,what is the correct condition ?
As per documentation, there is a method just for that: -&gt;isNull()

$query-&gt;isNull('myfield');
// Results in (myfield IS NULL)
$query-&gt;isNotNull('myfield');
// Results in (myfield IS NOT NULL)