Failed to extend disk : One of the parameters supplied is invalid. (1)

I'm in the process of extending a disk and I get this error "Failed to extend disk : One of the parameters supplied is invalid. (1)"

I'm in the .vmdk directory giving the absolute path the VM is off. I have also started the VM to ensure it is valid prior to shutting it down and trying the extend disk....

vmkfstools -X 8000m /vmfs/volumes/EMC_Store1/RHEL5_Test/RHEL5_Test.vmdk

thoughts?

Grant