Filing for my deceased Mom. What do I put for her spouse's "net income" on 2021 federal return, if I haven't yet filed his 2021 federal return? (He also died earlier 2021

Her husband died May 2021. She died September 2021. I'm the Executor of her Estate, trying to file her 2021 Final Return. It's asking me what her spouse's net income was on Line 23600 of his 2021 federal return. But we haven't filed his yet. So what do I write down?