sql server - update nvarchar not null field length -

i can update column's length if in null when created

mycolumn1 nvarchar(12) null 

my query:

alter table mytable alter column mycolumn1 nvarchar(13) null 

but how can change length if column not null?

mycolumn2 nvarchar(12) not null  alter table mytable alter column mycolumn2 nvarchar(13) not null 

when try sql server says

msg 4902, level 16, state 1, line 1
cannot find object "mycolumn2" because not exist or not have permissions.

i have permission because admin , table's column exist.

guys response... here table structure

create table [dbo].[mytable](     [mykey] [int] identity(1,1) not null,     [mycolumn1] [nvarchar](12) null,     [mycolumn2] [nvarchar](12) not null,   constraint [pk_mytable] primary key clustered  (     [mykey] asc )with (pad_index = off, statistics_norecompute = off, ignore_dup_key = off, allow_row_locks = on, allow_page_locks = on) on [primary] ) on [primary] 

column names case sensitive. please try

alter table mytable alter column [mycolumn2] nvarchar(13) not null 

i've tried , works fine on sqlserver2012.