I-140 RFE on experience certificate and that employer is unoperational

I got an RFE for I 140
USCIS recognizes that the Petitioner did provide letters from the beneficiary’s previous employers but the letters do not meet the regulatory requirements. The employer must issue the letter of experience on official letterhead and must list the employer’s name address, the date, the signer’s name and title, and a description of the beneficiary’s experience, including dates of employment and specific duties.

What if the previous employer is non operational? What documents should be submitted in this case?