sql stored procedure parameter problem

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SQL

Hello -
I have this weird problem with one of my SP's.
I have a SP and one of the parameters is nvarchar type.
and I am declaring the parameter, I am including value too, but when I am running there is no data returned.
short example:

@BookName nvarchar = null

Then in the where clause I have:

AND (o.BookName = @BookName OR @BookName IS NULL)

No data is returned.

But when I am doing:

AND (o.BookName = 'SQL Book' OR @BookName IS NULL)

I am getting the proper results.
Just to let you know that the field is no FK.

Any ideas what can be the reason?
Thanks