Azure VMSS with custom Image - update failed

Terraform
1

Hi,

TF 0.13.2 with latest AzureRM provider.

I provisioned a VMSS with custom Image. The problem is, if i know add datadisks to the VMSS, the update request fails with:

Error: Error updating LWindowsinux Virtual Machine Scale Set “vmss-apptf” (Resource Group “RG-TFCICDAPP105”): compute.VirtualMachineScaleSetsClient#Update: Failure sending request: StatusCode=400 – Original Error: Code=“InvalidParameter” Message=“Required parameter ‘imageReference’ is missing (null).” Target=“imageReference”

on vmss.tf line 5, in resource “azurerm_windows_virtual_machine_scale_set” “vmss”: 5: resource “azurerm_windows_virtual_machine_scale_set” “vmss” {​​​​​

Of course, imagereference is missing as I use source_image_id for the VMSS as document.

Any idea what´s causing the issue?