Problem with SQL Script - Cannot call methods on char

I have an update I am trying to do and I am getting Cannot call methods on char  

UPDATE imitmidx_sql
SET user_fld_9 = T.[CALIPER]
FROM
(
select distinct im.item_no,mu.[CALIPER]
from [001].[dbo].MarketingUpdates$ mu
inner join imitmidx_sql im on convert(char,(ltrim(rtrim(substring((im.item_no), (len(im.item_no)) - (len(im.item_no)-4), (len(im.item_no)-3) )))))= ltrim(rtrim(dbo.fn_StripCharacters(mu.[Part], '^a-z0-9'))) 
      WHERE mu.[CALIPER] <> im.user_fld_9
					) t
where convert(char,(ltrim(rtrim(substring((imitmidx_sql.item_no.item_no), (len(imitmidx_sql.item_no.item_no)) - (len(imitmidx_sql.item_no.item_no)-4), (len(imitmidx_sql.item_no.item_no)-3) )))) = ltrim(rtrim(t.item_no))

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