Nintex Forms Upgrade: The site with the id could not be found.

I am getting an error that has occurred now in two separate SharePoint 2013 farms while upgrading Nintex Forms 2013:

Upgrade-NFService : The site with the id d53b7029-e741-48ca-a9b7-ae78ace9b932

could not be found.

At line:1 char:81

+ ... entlyContinue; Upgrade-NFService}

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

+ CategoryInfo          : InvalidData: (Nintex.Forms.Sh...ommands.Upgrade:

Upgrade) [Upgrade-NFService], FileNotFoundException

+ FullyQualifiedErrorId : Nintex.Forms.SharePoint.PowerShell.Commands.Upgr

ade

Have you seen this error?