Communications Export Error

Getting following error when tried to export the Communications

What needs to be done to get the Communications exported ?

Traceback (most recent call last):
 File "/home/frappe/frappe-bench/apps/frappe/frappe/app.py", line 61, in application
 response = frappe.api.handle()
File "/home/frappe/frappe-bench/apps/frappe/frappe/api.py", line 54, in handle
return frappe.handler.handle()
File "/home/frappe/frappe-bench/apps/frappe/frappe/handler.py", line 21, in handle
data = execute_cmd(cmd)
File "/home/frappe/frappe-bench/apps/frappe/frappe/handler.py", line 52, in execute_cmd
return frappe.call(method, **frappe.form_dict)
File "/home/frappe/frappe-bench/apps/frappe/frappe/__init__.py", line 922, in call
return fn(*args, **newargs)
File "/home/frappe/frappe-bench/apps/frappe/frappe/core/page/data_import_tool/exporter.py", line 292, in get_template
xlsx_file = make_xlsx(reader, "Data Import Template")
File "/home/frappe/frappe-bench/apps/frappe/frappe/utils/xlsxutils.py", line 32, in make_xlsx
ws.append(clean_row)
File "/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/openpyxl/writer/write_only.py", line 236, in append
self.writer.send(row)
File "/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/openpyxl/writer/write_only.py", line 166, in _write_header
cell.value = value
File "/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/openpyxl/cell/cell.py", line 294, in value
self._bind_value(value)
File "/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/openpyxl/cell/cell.py", line 191, in _bind_value
value = self.check_string(value)
 File "/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/openpyxl/cell/cell.py", line 156, in check_string
raise IllegalCharacterError 
IllegalCharacterError

the illegal character might be the problem which is the reason the mail transfer connection was broken during my import
Connection was broken when connected to the server during import also

What are the characters that are not allowed