Custom Attribute mapping doesnt work for Nested Claims in ID Token.

If i have the ID token in the following format - i am not able to access the resource_access field.
{
"resource_access": {
"my_resource": {
"roles": [
"my_roles"
]
}
}

Here is my configuration for custom claim mapping :

spec:
authentication:
oidc:
attributeMapping:
resource_access: "assertion.resource_access"

Here is the error i am getting in the log :
Unable to process the request: The CEL expression for 'resource_access' did not return a string data type.