Check your state laws on probate. Because these laws are fairly uniform state-to-state, there is probably an abbreviated form of probate that the heirs can use. It may be as simple as signing an affidavit of heirship.
Because you said that there are little or no assets, I assume this check is not very large. Have someone call your bank's attorney to discuss how to handle this. Get it in writing--then file the answer in your procedures for the next time it happens.
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Better a patient man than a warrior, a man who controls his temper than one who takes a city