	AS OF SEPTEMBER 30,
	2019			2018
Trade accounts receivable	$	25,144		$	15,776
Allowance for doubtful accounts		(951	)		(1,453	)
	$	24,193		$	14,323

The allowance for doubtful accounts represents the Company’s best estimate of probable credit losses. The Company determines the allowance based upon historical experience and a current review of its accounts receivable balances. Accounts receivable balances are charged off against the allowance whenever it is probable that the receivable balance will not be recoverable.

Trade accounts receivable at September 30, 2019 includes $8.5 million due from an international seismic marine customer that, as of September 30, 2019 rented a significant amount of marine nodal equipment from the Company. The Company has experienced cash collection difficulties with this customer throughout fiscal year 2019 due to the customer’s inability to generate enough cash flow to pay its obligations in a timely manner. In November 2019, the Company accepted from the customer a plan to bring the Company’s current and future unpaid invoices to a satisfactory status. This plan contemplates completion during the Company’s second fiscal quarter ending March 31, 2020, and is premised upon the customer’s (i) projections of free cash flows from an existing contract with a third-party and (ii) potential access to capital transactions and/or future borrowing availability from its bank. While the Company has significant concerns about the ultimate collection of its accounts receivable from this customer, it has not, and does not currently intend to, provide any significant bad debt reserves toward its outstanding accounts receivable balance from this customer unless and until it becomes probable (in the Company’s judgement) that the customer cannot (i) generate free cash flows from its existing contract and (ii) complete capital transactions and/or borrow from its bank.

See Note 1 to these consolidated financial statements for information on concentrations of credit risk.

	AS OF SEPTEMBER 30,
	2019			2018
Promissory notes	$	780		$	5,646
Sales-type lease		2,692			5,533
Total financing receivables		3,472			11,179
Unearned income:
Promissory notes		—			(95	)
Sales-type lease		(55	)		(237	)
Total unearned income		(55	)		(332	)
Total financing receivables, net of unearned income		3,417			10,847
Allowance for doubtful promissory notes		—			(1,849	)
Less current portion		(3,233	)		(4,258	)
Non-current financing receivables	$	184		$	4,740

Promissory notes receivable are generally collateralized by the products sold, and bear interest at rates ranging up to 5% per year. The promissory notes receivable mature at various times through May 2021. The Company has, on occasion, extended or renewed notes receivable as they mature, but there is no obligation to do so.

The Company entered into a sales-type lease in September 2017 resulting from the sale of rental equipment. The sales-type lease has a term of three years. Future minimum lease payments required under the lease at September 30, 2019 were $2.9 million, including $0.1 million of unearned income. The future minimum lease payments are due in fiscal year 2020. The ownership of the equipment will transfer to the lessee at the end of the lease term.