Issue removing Nodes with swis api

PS C:\Users\212040597> Get-SwisData $swis "Select uri from Orion.Nodes Where caption='RD000D3A31B738'"

swis://opswusvmmona01./Orion/Orion.Nodes/NodeID=243

PS C:\Users\212040597> $uri = Get-SwisData $swis "Select uri from Orion.Nodes Where caption='RD000D3A31B738'"

PS C:\Users\212040597> $uri

swis://opswusvmmona01./Orion/Orion.Nodes/NodeID=243

PS C:\Users\212040597> Remove-SwisObject $swis $uri

Remove-SwisObject : Object reference not set to an instance of an object.

At line:1 char:1

+ Remove-SwisObject $swis $uri

+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~

    + CategoryInfo          : InvalidOperation: (:) [Remove-SwisObject], FaultException`1

    + FullyQualifiedErrorId : SwisError,SwisPowerShell.RemoveSwisObject

PS C:\Users\212040597>

What am I doing wrong? Ultimately I need to remove multiple nodes at once and I do know the criteria:

Select uri from Orion.Nodes where caption like 'RD%'

I will need to remove all nodes that being with RD... Any help is MUCH appreciated.