Invalid number shows in diagnostic > display database error when trying to save custom form in ebs 1

help,

subsidary code is an LOV  datatype varchar2

vendor site id  datatype varchar2

for rule seq 5

trigger event : WHEN-NEW-FORM-INSTANCE

processing mode : not in enter-query mode

for Actions seq 10

object type : global variable

targer object : xx_vendor_site_id_csa

property : initial value

value : "NULL"

for Actions seq 20

object type : global variable

targer object : xx_vendor_site_id_csa

property : VALUE

value : TO_CHAR(${item.site.vendor_site_id.value})

when trying to save its nothing happen then i checked diagnostic > display database error and its throws invalid number.

can anyone explain what should i do?