Backup of VM fails after enabling replica..

Hi,

I have hyper-v failover cluster running on 2012 R2,
and using DPM 2012 R2 to protects the vm's.

When i enabling the vm to replicate to another host,
that use a nas to Storage. The backup of the primary vm fails With this error:

Starting synchronization on \Online\VM1 failed:

Error 33343: DPM has detected that protection agents are
not installed on the following server(s) : NAS

Recommended action: 1) To install agents on these
servers, on the Agents tab in the Management task area, click Install in the
Action pane.

2) If any of the above servers correspond to a cluster or
a mirror, you need to install the DPM protection agent on all the physical
nodes of that cluster/mirror.

So why does the dpm need to have an agent on the storage
that the replicated vm is using? When I’m trying to backup the primary vm.