How can I stop adding NHibernate "or this._property is null" for generated queries?

Question

I use NHibernate to query my database, and one of the fields we work with was used to match nullable bool ( bool? )

We changed it to a regular bool, supported by the SQL NOT NOT NULL field.

NHibernate still generates queries that look like this:

 SELECT count(*) as y0_ FROM mydb.dbo.[Customer] this_ inner join mydb.dbo.[Order] fi1_ on this_.OrderId = fi1_.Id WHERE (this_.CustomerId = 9625 /* @p0 */ and (this_.Deleted = 0 /* @p1 */ or this_.Deleted is null)) 

This last line — or this_.Deleted is null — is unnecessary and actually slows down the resulting query, causing SQL Server to perform additional NULL checks.

How can I get NHibernate to create only this_.Deleted = 0 and leave the NULL check?