rds_instance allocated_storage works on 'create', but fails on 'present'

[benpillet]

If I don't specify the allocated_storage when the instance doesn't exist yet, I get this:
Error: Could not set 'present' on ensure: missing required parameter params[:allocated_storage]

If I specify the allocated_storage when the instance does exist, I get this:
Error: allocated_storage property is read-only once rds_instance created.

So, I end up commenting/uncommenting it depending on whether the instance exists or not. It's very clumsy. Is there a way to make allocated_storage throwing a warning if the instance exists and the size is a mismatch? or some other functionality that doesn't require changing the manifest depending on the resource existance?