Transact SQL :: Invalid Length Parameter Passed To Left Or Substring

is it possible to identify which value is causing me the above  error message and how to resolve it,These are for British postcodes.

create table #tmp (postcode varchar(200) NULL)
insert into #tmp values ('NULL')
insert into #tmp values ('-')
insert into #tmp values ('.')
insert into #tmp values ('0L6 7TP')
insert into #tmp values ('AB10 1WP')
insert into #tmp values ('AB51 5HH')

[code]...

This is the main query

select postcode,LEFT([Postcode], CHARINDEX(' ',[Postcode]) - 1)
from #tmp
order by Postcode
drop table #tmp