Select Rows From Database Where Fields Are Null

Hi, how do i do a select statement with asp.net to return a record if a field is null.  I have the following code:

SqlCommand cmd = new SqlCommand("SELECT * FROM Categories WHERE ParentId = @ParentId", cn);
.Parameters.Add("@ParentId", SqlDbType.Int).Value = parentId != null ? (object) parentId : DBNull.Value;
cn.Open();

The variable parentId is a nullable int.  However when i pass in a null value it doesn't return anything even though there are records in the database that have a null value for the ParentId field.

Appreciate if someone could tell me what i am doing wrong.  Thanks