拼接字符串发生的错误

declare @iyear int
declare @imonth int

set @iyear=2019
set @imonth =9

declare @ddate as nvarchar
declare @ddate1 as nvarchar(100)

print cast(@iyear as nvarchar)+'-'+cast(@imonth as nvarchar)+'-01'

set @ddate=cast(@iyear as nvarchar)+'-'+cast(@imonth as nvarchar)+'-01'
set @ddate1=cast(@iyear as nvarchar)+'-'+cast(@imonth as nvarchar)+'-01'
print @ddate
print @ddate1

　　

2019-9-01
2
2019-9-01