PS C:\Users\slee> $x = 1 | Select-Object -Property @{Name = 'X'; Expression = {}}
PS C:\Users\slee> $x | Group-Object X

Count Name                      Group
----- ----                      -----
    1                           {@{X=}}

Group-Object : Object reference not set to an instance of an object.
At line:1 char:6
+ $x | Group-Object X
+      ~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [Group-Object], NullReferenceException
    + FullyQualifiedErrorId : System.NullReferenceException,Microsoft.PowerShell.Commands.GroupObjectCommand

PS C:\Users\slee> $x = 1 | Select-Object -Property @{Name = 'X'; Expression = {$null}}
PS C:\Users\slee> $x | Group-Object X

Count Name                      Group
----- ----                      -----
    1                           {@{X=}}

PS C:\Users\slee> $psversiontable

Name                           Value
----                           -----
GitCommitId                    v6.0.0-alpha.17
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
CLRVersion
SerializationVersion           1.1.0.1
PSEdition                      Core
BuildVersion                   3.0.0.0
PSVersion                      6.0.0-alpha
WSManStackVersion              3.0
PSRemotingProtocolVersion      2.3