Interoperability Dell OS 9.13 and Cisco

We just acquired new Dell switches and I am attempting to incorporate them into our Cisco infrastructure. Not working as I had hoped and I am seeking some advice.

I am attempting to network 2 stacked Dell S4048 switches running OS 9.13 to a Cisco 4900M  running 15.0(2)SG.

I have set up a 4 port LAG connection between the two and everyone is happy. But the Dell is running RSTP while the Cisco is running PVST. Actually, all our switches are Cisco and all are running PVST. Since the Dell has PVST support by a 3rd party, I am attempting to use that spanning tree protocol.

The problem is when I switch the Dell spanning tree to PVST, the Dell thinks it's the Root Bridge, no surprise there. But BPDU packets are not arriving across the LAG (po1), at least Dell does not recognize the packets perhaps? On the Cisco switch spanning tree is enabled on po1.

So the Dell never does find out who is really the Root Bridge. Could be the answer here but, VLAN 10 is the Native VLAN and also our VLAN used to connect to devices. It is VLAN 10 that is failing as Dell thinks it's the Root for VLAN 10. And it is not.

The question; Is Dell really compatible with Cisco PVST+ spanning tree protocol? Is there something that must be done to have Dell recognize the BPDU messages from the Cisco switch?

Ideas?