Outgoing Email error message

I have looked through the other topics and didn’t find this problem. I am experiencing problems with sending email from a Purchase order. Is says that I have a server error and gives me the following code. Any help will be greatly appreciated.

   Traceback (innermost last):
  File "/home/erpnext/frappe-bench/apps/frappe/frappe/app.py", line 51, in application
    response = frappe.handler.handle()
  File "/home/erpnext/frappe-bench/apps/frappe/frappe/handler.py", line 69, in handle
  File "/home/erpnext/frappe-bench/apps/frappe/frappe/handler.py", line 92, in execute_cmd
    ret = frappe.call(method, **frappe.form_dict)
  File "/home/erpnext/frappe-bench/apps/frappe/frappe/__init__.py", line 751, in call
    return fn(*args, **newargs)
  File "/home/erpnext/frappe-bench/apps/frappe/frappe/core/doctype/communication/communication.py", line 259, in make
    "recipients": ", ".join(recipients) if recipients else None
 TypeError: sequence item 0: expected string or Unicode, NoneType found

Seems like an invalid email address, fixed the error message.