Empty variable Get-SPSiteAdministration DiskUsed

I`m writing script, and i get empty results.

$SitesByte =   (Get-SPSiteAdministration -Identity http://siteurl ).DiskUsed
When i see output $SitesByte is empty.
When i run  Get-SPSiteAdministration -Identity http://siteurl, i get output.
I even type entire command if there are any hidden character, but result is the same. I`m missing or something is staring me directly in my face.
I use SharePoint 2013