K-1 in Trust – Beneficiary won't give social security number

I am the trustee of a trust trying to complete the final 1041 tax return and two of the beneficiaries will not give me their Social Security information in order to complete the K-1.  Being a trustee is one of the most thankless jobs I've ever had as this just further delays and complicates my finishing of the job. 🙄

 

I've read in these forums that I might need to attach a signed affidavit of some sort to the actual return, does anyone have any experience with this and what this document should say?  Thanks for any help