Value is empty and 'isEmpty' function cannot be applied

Value is empty and 'isEmpty' function cannot be applied

Hello,

I am having a very strange error in my custom  function with Deluge. It is a function that creates. invoices automatically every Friday if the sales order is shipped. I added a verification to know if the customer has credit notes actives. If the costumer has some credits, the invoice is not created. 
The thing is I am receiving the error you see in the title "Check and update the code in line 76 as there is a Exception : Value is empty and 'isEmpty' function cannot be applied"

  2. urlcredit = "https://books.zoho.eu/api/v3/creditnotes?organization_id=" + organizationID + "&customer_id=" + customerID + "&status=open";
  3. credit_note = invokeurl
  4. [
  5. url :urlcredit
  6. type :GET
  7. connection:"v2_books_conpas"
  8. ];
  9. comp_tienecredito = credit_note.get("creditnotes");
  10. if(comp_tienecredito.isEmpty() == false)
  11. {
  12. tieneCredito = true;
  13. numeroCreditos = numeroCreditos + 1;
  14. }