I got this error "Conversion failed when converting the varchar value 'A1' to data type int." when I am trying to select varchar valure using stored procedure

ALTER PROCEDURE [dbo].[InsertAlbum_sp]
		(
			@Member_Id varchar(10),
			@albumTitle varchar(50)
		)

AS
BEGIN
	-- SET NOCOUNT ON added to prevent extra result sets from
	-- interfering with SELECT statements.
	SET NOCOUNT ON;
	declare @newId varchar(10)
    declare @length int
	if(select MAX(album_Id) from album_tb)<>'null'
	begin
	
		set @length=(select len(max(album_Id)) from album_tb)
		set @newId=(select SUBSTRING(max(album_Id),1,@length)+1 from album_tb)
		set @newId='A'+RIGHT(cast(@newId as varchar(10)),1)
	end
	else
	begin
		set @newId='A1'
	end

	insert into album_tb (album_Id,Member_Id,albumTitle) values (@newId,@Member_Id,@albumTitle)
	select @newId
END