Fixed in v6.1.0
See help about_Properties:
Beginning in Windows PowerShell 3.0, Windows PowerShell tries
to prevent scripting errors that result from the differing
properties of scalar objects and collections.
-- If you request the Count or Length property of zero objects
or of one object, Windows PowerShell returns the correct value.
This is not the case for PSCustomObject objects.
They do not have surrogate Count and Length.
Test-1.normal.vs.custom.ps1 shows that "normal"
objects have the surrogate Count and "custom" do not.
Test-2.Select-Object.ps1 reminds that custom objects
may appear even if you do not create them explicitly. For example, Select-Object
may return custom objects. So the result Count may depend on using or not using
Select-Object in a pipeline.