Update-AzEventGridSystemTopicEventSubscription fails when setting -DeliveryWithResourceIdentityType 'None'

[narannakata]

Description

I am encountering an issue where Update-AzEventGridSystemTopicEventSubscription fails to remove the Managed Identity from an Event Subscription.
According to the Update-AzEventGridSystemTopicEventSubscription (Az.EventGrid) | Microsoft Learn, the parameter -DeliveryWithResourceIdentityType accepts 'None' to remove any identity. However, executing this command results in a type conversion error.
It appears that the underlying Enum EventSubscriptionIdentityType does not accept None, despite the cmdlet documentation stating otherwise.

Issue script & Debug output

PS C:\Windows\system32> $DebugPreference='Continue'
PS C:\Windows\system32> Update-AzEventGridSystemTopicEventSubscription `
>>  -ResourceGroupName $resourceGroupName `
>>  -SystemTopicName $eventGridSysTopicName `
>>  -EventSubscriptionName $eventSubscriptionName `
>>  -DeliveryWithResourceIdentityDestination $destination `
>>  -DeliveryWithResourceIdentityType 'None' `
>>  -ErrorAction Stop
Debug: 13:29:23 - [ConfigManager] Got nothing from [DisplaySecretsWarning], Module = [], Cmdlet = []. Returning default value [True].
Debug: 13:29:23 - [ConfigManager] Got nothing from [DisplaySecretsWarning], Module = [], Cmdlet = []. Returning default value [True].                                                     
Debug: 13:29:23 - GetAzureRMContextCommand begin processing with ParameterSet 'GetSingleContext'.                                                                                         
Debug: 13:29:23 - [ConfigManager] Got nothing from [DisplayBreakingChangeWarning], Module = [Az.Accounts], Cmdlet = [Get-AzContext]. Returning default value [True].                      
Debug: 13:29:23 - [ConfigManager] Got nothing from [DisplaySecretsWarning], Module = [], Cmdlet = []. Returning default value [True].                                                     
Debug: 13:29:23 - [ConfigManager] Got nothing from [DisplayRegionIdentified], Module = [], Cmdlet = []. Returning default value [True].                                                   
Debug: 13:29:23 - [ConfigManager] Got nothing from [CheckForUpgrade], Module = [], Cmdlet = []. Returning default value [True].
Debug: 13:29:23 - GetAzureRMContextCommand end processing.
Debug: [CmdletBeginProcessing]: Starting command
Debug: CmdletBeginProcessing:
Debug: CmdletProcessRecordStart:
Debug: CmdletGetPipeline:
Debug: CmdletBeforeAPICall:
Debug: URLCreated:
/subscriptions/***/resourceGroups/test-east/providers/Microsoft.EventGrid/systemTopics/narantestaccount-2b04123a-9355-4263-85a8-e8f8727a35c2/eventSubscript
ions/StorageAntimalwareSubscription?api-version=2023-06-01-preview
Debug: RequestCreated:
/subscriptions/***/resourceGroups/test-east/providers/Microsoft.EventGrid/systemTopics/narantestaccount-2b04123a-9355-4263-85a8-e8f8727a35c2/eventSubscript
ions/StorageAntimalwareSubscription?api-version=2023-06-01-preview
Debug: HeaderParametersAdded:
Debug: BodyContentSet:
Debug: 13:29:23 - [ConfigManager] Got nothing from [DisableInstanceDiscovery], Module = [], Cmdlet = []. Returning default value [False].
Debug: 13:29:23 - [ConfigManager] Got nothing from [EnableLoginByWam], Module = [], Cmdlet = []. Returning default value [True].
Debug: ============================ HTTP REQUEST ============================

HTTP Method:
PATCH

Absolute Uri:
https://management.azure.com/subscriptions/***/resourceGroups/test-east/providers/Microsoft.EventGrid/systemTopics/narantestaccount-2b04123a-9355-4263-85a8 -e8f8727a35c2/eventSubscriptions/StorageAntimalwareSubscription?api-version=2023-06-01-preview                                                                                                                                                                                                                                                                                            Headers:                                                                                                                                                                                     x-ms-unique-id                : 5                                                                                                                                                            x-ms-client-request-id        : 7738d1cc-1f17-47c3-a7ca-8b996c883ebf                                                                                                                         CommandName                   : Update-AzEventGridSystemTopicEventSubscription                                                                                                               FullCommandName               : Update-AzEventGridSystemTopicEventSubscription_UpdateExpanded
ParameterSetName              : __AllParameterSets
User-Agent                    : AzurePowershell/v0.0.0,PSVersion/v5.1.26100.7824,Az.EventGrid/2.1.0

Body:
{
  "deliveryWithResourceIdentity": {
    "identity": {
      "type": "None"
    },
    "destination": {
      "endpointType": "StorageQueue",
      "properties": {
        "resourceId": "/subscriptions/***/resourceGroups/test-east/providers/microsoft.storage/storageaccounts/narantestaccount",
        "queueName": "myqueue"
      }
    }
  }
}

Debug: BeforeCall:
Debug: ============================ HTTP RESPONSE ============================

Status Code:
BadRequest

Headers:
Pragma                        : no-cache
Strict-Transport-Security     : max-age=31536000; includeSubDomains
x-ms-operation-identifier     : tenantId=***,objectId=a7e22cff-3860-4622-b344-52db901d3644/japaneast/28227b51-97a6-488a-b83c-2db05132f116
x-ms-ratelimit-remaining-subscription-writes: 199
x-ms-ratelimit-remaining-subscription-global-writes: 2999
x-ms-request-id               : 426aeda7-2fa3-4a46-81a8-c1fe3a85e46c
x-ms-correlation-request-id   : 426aeda7-2fa3-4a46-81a8-c1fe3a85e46c
x-ms-routing-request-id       : JAPANEAST:20260224T042928Z:426aeda7-2fa3-4a46-81a8-c1fe3a85e46c
X-Content-Type-Options        : nosniff
X-Cache                       : CONFIG_NOCACHE
X-MSEdge-Ref                  : Ref A: 76940B9A29D04A3DBD552E83AF195A4F Ref B: SG2AA1070303036 Ref C: 2026-02-24T04:29:28Z
Cache-Control                 : no-cache
Date                          : Tue, 24 Feb 2026 04:29:28 GMT

Body:
{
  "error": {
    "code": "InvalidRequest",
    "message": "Error converting value \"None\" to type 'Microsoft.Azure.Events.ResourceProvider.Common.Contracts.EventSubscriptionIdentityType'. Path
'deliveryWithResourceIdentity.identity.type', line 1, position 58."
  }
}

Debug: ResponseCreated:
Debug: BeforeResponseDispatch:
Debug: CmdletProcessRecordEnd:
Update-AzEventGridSystemTopicEventSubscription : Error converting value "None" to type 'Microsoft.Azure.Events.ResourceProvider.Common.Contracts.EventSubscriptionIdentityType'. Path 'deliv
eryWithResourceIdentity.identity.type', line 1, position 58.
発生場所 行:1 文字:1
+ Update-AzEventGridSystemTopicEventSubscription `
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: ({ }:<>f__AnonymousType1) [Update-AzEventG..._UpdateExpanded]、RestException
    + FullyQualifiedErrorId : InvalidRequest,Microsoft.Azure.PowerShell.Cmdlets.EventGrid.Cmdlets.UpdateAzEventGridSystemTopicEventSubscription_UpdateExpanded

Environment data

PS C:\Windows\system32> $PSVersionTable

Name                           Value
----                           -----
PSVersion                      5.1.26100.7824
PSEdition                      Desktop
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
BuildVersion                   10.0.26100.7824
CLRVersion                     4.0.30319.42000
WSManStackVersion              3.0
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1

Module versions

PS C:\Windows\system32> Get-Module Az*

ModuleType Version    Name                                ExportedCommands
---------- -------    ----                                ----------------
Script     5.3.0      Az.Accounts                         {Add-AzEnvironment, Clear-AzConfig, Clear-AzContext, Clear-AzDefault...}
Script     2.2.0      Az.EventGrid                        {Disable-AzEventGridPartnerTopic, Enable-AzEventGridPartnerDestination, Enable-AzEventGridPartnerTopic, Get-AzEventGridCaCertif...

Error output