Approval Process in Customer invoice

Hello , 

I need to prevent the creation of a new customer invoice if the customer is not approved .

I tried to create a custom event to control that , But it does’t stop  the creation of a new customer invoice .

I applied the custom event on CustomerOrderInvHead (LU) and INVOICE_TAB and here is the script of my event action : 

DECLARE 
lu_name_ VARCHAR2(1000) ; 
key_ref_  VARCHAR2(32000) ; 
result_ VARCHAR2(32000) ; 
--approval_status_ VARCHAR2(1000) ; 
BEGIN
lu_name_     := 'CustomerOrderInvHead'; 
key_ref_   := 'CUSTOMER_ID=&NEW:IDENTITY^'  ;                     
result_ := approval_routing_api.is_approved(lu_name_,key_ref_);    

IF (result_ = 'FALSE') THEN
      Error_SYS.Record_General(lu_name_, 'APPROVAL_ROUTING: This invoice cannot be created. The customer is not approved') ;
end if;

END;

Does anyone have an idea what could be the problem ? 

Thank you ,

Hanane