Is it possible to send me the raw output from this cluster? This isn't necessarily a 9.1P2 issue, other than the possibility of the property name being changed. The real issue is that this condition shouldn't produce a terminating error. I will send you an email offline to discuss further.
We are about ready to release a 3.5.0 version, however this error you are seeing is specific to the system you are pulling from. Would it be possible to get the raw data from the Get-NtapClusterData function for this system for troubleshooting?