How do i create an access policy for my azure function with bicep?

I have a resource defined in my bicep file like this below, these are two of the resources in my file, i deploy an azure function with the test_resource below, this works fine.

resource test_resource 'Microsoft.Web/sites@2021-03-01' = {
  name: resourceName
  location: location
  kind: 'functionapp'
  identity: {
    type: 'SystemAssigned'
  }
  properties: {
    httpsOnly: true
    serverFarmId: appServicePlan_ResourceId
  }
}

and i am attempting to create an access policy as shown below, however i get an error regard the objectId, is there a way to configure the access policy for the above resource, perharps i am passing the wrong id in

"Invalid value found at accessPolicies[0].ObjectId: 

but i am passing the test_resource.id as shown in the keyvault_access_policy resource definition.

resource devops_keyvault 'Microsoft.KeyVault/vaults@2021-10-01' existing = {
  name: keyVaultName
}

resource keyvault_access_policy 'Microsoft.KeyVault/vaults/accessPolicies@2021-10-01' = {
  name: 'add'
  parent: devops_keyvault
  properties: {
    accessPolicies: [
      {
        objectId: test_resource.id 
        permissions: {
          'keys': []
          'secrets': [
            'list'
            'get'
          ]
          'certificates': [
            'list'
            'get'
          ]
        }
        tenantId: subscription().tenantId
      }
    ]
  }
}