MySQL - Illegal mix of collations (utf8_general_ci,IMPLICIT) and (utf8_unicode_ci,IMPLICIT) for operation

 select  customers_email_address,customers_id  from   customers_backup as a , rjin_customer as c where c.email=a.customers_email_address

Illegal mix of collations (utf8_general_ci,IMPLICIT) and (utf8_unicode_ci,IMPLICIT) for operation '='