Exchange 2013 anti-Malware - set new policy to default fails with Dr Watson

I've successfully created a new anti-malware policy via Powershell but I can't make it set to default

I've tried

Set-MalwareFilterPolicy -identity "Custom Policy" -makedefault

Set-MalwareFilterPolicy -identity "Custom Policy" -makedefault:$true

Set-MalwareFilterPolicy -identity "Custom Policy" -makedefault $true

all three generate a Dr Watson and the following error:

"WARNING: An unexpected error has occurred and a Watson dump is being generated: Unable to cast object of type
'Microsoft.Exchange.Data.Directory.SystemConfiguration.ADTopologyConfigurationSession' to type
'Microsoft.Exchange.Data.Directory.SystemConfiguration.ITenantConfigurationSession'.
Specified cast is not valid.
    + CategoryInfo          : NotSpecified: (:) [Set-MalwareFilterPolicy], InvalidCastException
    + FullyQualifiedErrorId : System.InvalidCastException,Microsoft.Exchange.Management.SystemConfigurationTasks.SetMa
   lwareFilterPolicy
    + PSComputerName        : blah.blah.com"

How the XXXX do I set this as the default in Powershell. Yes I can do it in the GUI but I need the powershell command

TIA

Tony