Azure Function gets null when Sourcing Application Settings from Key Vault when using consumption plan

Azure Function gets null when Sourcing Application Settings from Key Vault

I’m trying to use feature
Sourcing Application Settings from Key Vault
Motioned here
https://azure.microsoft.com/en-us/blog/simplifying-security-for-serverless-and-web-apps-with-azure-functions-and-app-service/

But I keep getting null exception when I replace the value with key vault reference in the application settings.
Here’s the steps I take
1. Turn on System assigned identity for my azure function
2. Add access policy on key vault to allow System assigned identity have full access to the key vault.

3. Replace settings in application settings with key vault reference( try both secreturi and vaultname version)
And then I can only get following exception during start up
**Value cannot be null. Parameter name: uriString**

I tried the same thing for user assigned identity and same result.

Provider more information here.
I created another azure function in app service plan and found the feature
Sourcing Application Settings from Key Vault
Works correctly there.

Is that expected Sourcing Application Settings from Key Vault not working for consumption plan?