Opening Invoice Tool Error Please select Customer first

I’ve been trying to use the Opening Invoice tool to record the sales invoices for my accounts receivable opening balance. i have used it with purchase invoices with out any problem but when i try it with sales invoice type, i get an error “Please select customer first”. I have selected the correct customer as my entry for the party field but the error persist.

Here is the error from the log:

“items”: [
“uom”: “Nos”,
“rate”: 114877.39,
“qty”: “1”,
“conversion_factor”: 1.0,
“item_name”: “Opening Invoice Item”,
“description”: “Opening Invoice Item”,
“income_account”: "191000 - Temporary Opening ",
“cost_center”: “Main”,
“doctype”: “Sales Invoice Item”
“is_opening”: “Yes”,
“set_posting_time”: 1,
“company”: “My Company”,
“cost_center”: null,
“due_date”: “2021-07-27”,
“posting_date”: “2021-07-27”,
“customer”: null,
“is_pos”: 0,
“doctype”: “Sales Invoice”,
“update_stock”: 0,
“department”: null,
“branch”: null,
“currency”: “PHP”,
“letter_head”: null

Traceback (most recent call last):
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/doctype/opening_invoice_creation_tool/”, line 203, in start_import
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/”, line 237, in insert
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/”, line 960, in run_before_save_methods
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/”, line 858, in run_method
out = Document.hook(fn)(self, *args, **kwargs)
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/”, line 1147, in composer
return composed(self, method, *args, **kwargs)
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/”, line 1130, in runner
add_to_return_value(self, fn(self, *args, **kwargs))
File “/home/frappe/frappe-bench/apps/frappe/frappe/model/”, line 852, in
fn = lambda self, *args, **kwargs: getattr(self, method)(*args, **kwargs)
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/doctype/sales_invoice/”, line 80, in validate
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/doctype/sales_invoice/”, line 160, in set_tax_withholding
tax_withholding_details = get_party_tax_withholding_details(self)
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/doctype/tax_withholding_category/”, line 33, in get_party_tax_withholding_details
party_type, party = get_party_details(inv)
File “/home/frappe/frappe-bench/apps/erpnext/erpnext/accounts/doctype/tax_withholding_category/”, line 26, in get_party_details
frappe.throw(_(“Please select {0} first”).format(party_type))
File “/home/frappe/frappe-bench/apps/frappe/frappe/”, line 427, in throw
msgprint(msg, raise_exception=exc, title=title, indicator=‘red’, is_minimizable=is_minimizable, wide=wide, as_list=as_list)
File “/home/frappe/frappe-bench/apps/frappe/frappe/”, line 406, in msgprint
File “/home/frappe/frappe-bench/apps/frappe/frappe/”, line 360, in _raise_exception
raise raise_exception(msg)
frappe.exceptions.ValidationError: Please select Customer first

As you can see, the customer is “null”. I don’t know how to fix this.