mysql FUNCTION CONCAT does not exist

[jeorg calimet]

I am on mysql 5.1

if I do SELECT CONCAT (a,b) FROM tb it works
If I do

UPDATE jos_vm_product_files pf
INNER JOIN jos_vm_product_imgs pi
ON pf.file_product_id = pi.product_id
SET pf.file_title= concat ( replace(pi.cat_fr , '_', ' '),' ', replace(pi.product_name, '_', ' ') );

I get FUNCTION mydatabase.CONCAT does not exist

it works perfectly on my localhost

any idea ?

thank you