Is this a normal request from seller?

[Cong Vu]

This is my first rental property so obviously I am a newbie and just starting out. I appreciate all the inputs from you all.

I am buying this property which has a tenant in it. The owner sent me a lease agreement which was signed about 8 years ago and the lease is month-to-month basis. However, it does not spell out in the lease and it is per the seller's words. I want to have the tenant sign the estoppel agreement to verify the validity of the seller's claims but the seller adamantly doesn't want me to talk to the tenant at all. He requested that all questions direct to the listing agent and he doesn't want either me or my agent to talk to the tenant. 

Is this normal? should I go ahead with the deal? what should I do to protect myself? 

Thank you in advance!