逾期付款的委婉提醒技巧 — 国际贸易填空题

在撰写首封催款函时，为体现委婉提醒技巧，通常将逾期原因归于无意疏忽而非客户失信。请补全下函中空缺处的委婉表达："Dear Sirs, We wish to draw your attention to Invoice No. 302 for USD 8,500, which fell due on May 10 and remains unpaid. As no remittance has reached us, we assume this is simply an ____ on your part, and we would appreciate it if you could settle the account at your earliest convenience." 此处应填入的单词是 ____。