Azure DSC validation error on portal

Any ideas on the cause value for the template parameter 'modulesUrl' at line '11' and column '20' is not provided returned when validating a new VM in the Azure Portal with the DSC extension. It's no help knowing that some underlying template has errors.

DSC config stripped to a minimum test version is

configuration WebServer {
    Node localhost {

        WindowsFeature IIS {
            Ensure = 'Present'
            Name = 'Web-Server'
        }
        WindowsFeature DotNet {
            Ensure = 'Present'
            Name = 'Web-Asp-Net45'
            DependsOn = '[WindowsFeature]IIS'
        }

        File Namer {
            DependsOn = '[WindowsFeature]DotNet'
            DestinationPath = 'C:\InetPub\wwwroot\name.aspx'
            Type = 'File'
            Contents = '<h1>My Name is: <%=Environment.MachineName %></h1><hr />'
        }
    }

}

I thought that dumb aspx line might be troubling it but even if i replace that with just 'horses' it still throws this error. I'm using dsc.ps1\WebServer as the Module-Qualified name and have tried both the zip from Publish-AzureRMVMDscConfiguration -OutputArchivePath and the plain ps1 file. Tried version 2.15, 2.21 (as per an example in the documentation - its unclear that this refers to the Dsc version). This is in the West-Europe region, creating a DSv2 Windows Server 2016 Core image.

Update: That DSC script works on a local Hyper-V 2016 Core VM and correctly deploys name.aspx so I don't think the problem is there.