Nikken erected a family gravestone in a Zen cemetery and conducted a service there on July 17, 1990, even though Nichiren Daishonin himself was especially critical of Zen.
Nikko Shonin strictly instructed believers never to visit heretical sects. From the 26 Admonitions of Nikko Shonin (Article 6):
"The believers of the Fuji School are forbidden to visit heretical temples or shrines, not even on the excuse they are merely sightseeing. Otherwise, they will receive a share of the hobo (slander) committed at these places. It is regrettable to see them fooled by the devils dwelling in such places. This is not my own teaching; I am simply following the Gosho and the Sutra." [Gosho Zenshu, p. 1617]And Article 21:
"You should not sit with slanderers at religious ceremonies for fear of sharing their sin." [Gosho Zenshu, p. 1618]What reason did Nikken give for going against the teachings of Nichiren Daishonin? He said "That is a public cemetery." and "I erected it on the request of the family."