	AS OF SEPTEMBER 30,
	2020			2019
Trade accounts receivable	$	14,090		$	25,144
Allowance for doubtful accounts		(496	)		(951	)
	$	13,594		$	24,193

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.

The following table summarizes changes in the Company’s allowance for doubtful accounts for the fiscal year ended September 30, 2020 (in thousands):

Balance at October 1, 2019	$	951
Bad debt expense, net of recoveries		63
Write-offs		(518	)
Balance at September 30, 2020	$	496

In fiscal year 2020, the Company entered into an agreement with an international seismic marine customer to transition $13.0 million of unpaid invoices and late fees into a senior secured bond having a face value of $13.0 million. The bond was issued to the Company in July 2020. The customer had 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 and through the third quarter of fiscal year 2020 due to the customer’s inability to generate sufficient cash flows to pay its obligations in a timely manner. During fiscal year 2020, the Company recorded an $8.0 million reversal of an operating lease receivable resulting in an $8.0 million charge to rental revenue to reduce to zero the carrying value of an operating lease receivable owed by the customer. During fiscal year 2020, the Company received cash payments of $8.1 million from this customer. Since the inception of the lease during fiscal year 2018, the Company has received cash payments in excess of $24 million from this customer. See Note 4 – Investment in Debt Security for more information on the Company’s senior secured bond.

	AS OF SEPTEMBER 30,
	2020			2019
Promissory notes	$	184		$	780
Sales-type lease		—			2,692
Total financing receivables		184			3,472
Unearned income:
Sales-type lease		—			(55	)
Total unearned income		—			(55	)
Total financing receivables, net of unearned income		184			3,417
Less current portion		(184	)		(3,233	)
Non-current financing receivables	$	—		$	184

Promissory notes receivable are generally collateralized by the products sold, and bear interest at rates ranging up to 7% per year. The promissory notes receivable (including the unrecognized $10.0 million promissory note receivable disclosed below) mature at various times through January 2023. The Company has, on occasion, extended or renewed notes receivable as they mature, but there is no obligation to do so.

During the second quarter of fiscal year 2020, the Company partially financed a $12.5 million product sale by entering into a $10.0 million secured promissory note with a customer. The note has a three-year term and bears interest at 7% per year. Principal and interest payments of $0.3 million are due monthly until maturity. Due to the financial condition of the customer, the note receivable is not reflected on the Company’s consolidated balance sheet due to collectability concerns. See Note 2 for more information on this matter.

The Company entered into a sales-type lease in September 2017 resulting from the sale of rental equipment. The sales-type lease had a term of three years. At September 30, 2020, the Company had received all payment obligations required under the lease and ownership of equipment was transferred to the lessee.