Update Manager and ESXi 5.0 to 5.1

Hi

I am trying to upgrade my host to ESXi 5.1 using Update Manager running under ESXi 5.0u1 and vCenter 5.0u1.

When I import the ESXi 5.1 VIM ISO, I get the following error...

Import ESXi image
VCENTER
The uploaded upgrade package cannot be used with
VMware vSphere Update Manager.

Any ideas?

Thanks.