"Parameter 'osProfile' is not allowed" on attempt to create VMSS from a gallery image in C# - what am I doing wrong?

I am trying to create VM scale set using an existing galery image version (specialized) like this:

using Microsoft.Azure.Management.Fluent;
using Microsoft.Azure.Management.Compute.Fluent;
...
IGalleryImageVersion image = ...
IVirtualMachineScaleSet vmss = await Azure.VirtualMachineScaleSets
    .Define(vmssName)
    .WithRegion(region)
    .WithExistingResourceGroup(rgName)
    .WithSku(sku)
    .WithExistingPrimaryNetworkSubnet(network, subnetName)
    .WithoutPrimaryInternetFacingLoadBalancer()
    .WithoutPrimaryInternalLoadBalancer()
    .WithLinuxGalleryImageVersion(image.Id)
    .WithRootUsername(userName)
    .WithSsh(publicKey)
    .WithNewDataDisk(dataDiskSize)
    .WithCapacity(intendedSize)
    .WithExistingUserAssignedManagedServiceIdentity(identity)
    .WithVirtualMachinePublicIp()
    .CreateAsync(token);

What am I doing wrong? I am getting this exception:

Microsoft.Rest.Azure.CloudException
  HResult=0x80131500
  Message=Parameter 'osProfile' is not allowed.
  Source=Microsoft.Azure.Management.Compute.Fluent
  StackTrace:
   at Microsoft.Azure.Management.Compute.Fluent.VirtualMachineScaleSetsOperations.<BeginCreateOrUpdateWithHttpMessagesAsync>d__27.MoveNext()
   at Microsoft.Azure.Management.Compute.Fluent.VirtualMachineScaleSetsOperations.<CreateOrUpdateWithHttpMessagesAsync>d__5.MoveNext()
   at Microsoft.Azure.Management.Compute.Fluent.VirtualMachineScaleSetsOperationsExtensions.<CreateOrUpdateAsync>d__0.MoveNext()
   at Microsoft.Azure.Management.Compute.Fluent.VirtualMachineScaleSetImpl.<CreateInnerAsync>d__474.MoveNext()
   at Microsoft.Azure.Management.ResourceManager.Fluent.Core.GroupableParentResource`8.<CreateResourceAsync>d__5.MoveNext()
   at Microsoft.Azure.Management.ResourceManager.Fluent.Core.ResourceActions.Creatable`4.<Microsoft-Azure-Management-ResourceManager-Fluent-Core-ResourceActions-IResourceCreator<IResourceT>-CreateResourceAsync>d__15.MoveNext()
   at Microsoft.Azure.Management.ResourceManager.Fluent.Core.DAG.CreatorTaskItem`1.<ExecuteAsync>d__6.MoveNext()
   at Microsoft.Azure.Management.ResourceManager.Fluent.Core.DAG.TaskGroupBase`1.<ExecuteNodeTaskAsync>d__14.MoveNext()

I can create VMSS using a standard Linux image from marketplace. But I need to create VMSS with my own image from my own gallery. Note that the Fluent API does not give an option to omit root user name/ssh key from the call.