Request Help For Debugging A SQL Error

The dynamic SQL at the buttom causes a error: 
Msg 402, Level 16, State 1, Line 9
The data types ntext and nvarchar are incompatible in the equal to operator.
 
I think it is due to the one of the where conditions [URL] = @old_URL ,both are set to NULL(@URL=NULL, @old_URL=NULL).
How do I make changes to the SQL statement to compare the conditions where NULL comparsion is inevitable.
 exec sp_executesql N'UPDATE [Announcement] SET [Atype] = @Atype, [Title] = @Title, [Content] = @Content,  [URL] = @URL WHERE [AnnouncementID] = @old_AnnouncementID AND [Atype] = @old_Atype AND [Title] = @old_Title AND [Content] = @old_Content  AND [URL] = @old_URL',N'@Atype nvarchar(3),@Title nvarchar(3),@Content nvarchar(41),@URL nvarchar(4000),@old_AnnouncementID decimal(1,0),@old_Atype nvarchar(3),@old_Title nvarchar(3),@old_Content nvarchar(41),@old_URL nvarchar(4000)',@Atype=N'CMC',@Title=N'aaa',@Content=N'asdfg',@URL=NULL,@old_AnnouncementID=6,@old_Atype=N'CMC',@old_Title=N'aaa',@old_Content=N'asdfg',@old_URL=NULL 
Any help would be apprecaied,
Ricky.