PDF created with pdfrw addpage by copying from other pdf has no root?

Question

I have a code:

pdf = pdfrw.PdfReader(pdf_template)
pdfs= PdfWriter()

for page in pdf.pages:
   ### here some page manipulation
   pdfs.addpage(page)

pdfs.Root.AcroForm.update(pdfrw.PdfDict(NeedAppearances=pdfrw.PdfObject('true')))

I am getting error:

PdfWriter has no Root