Unable to use expressions to add to string set in DynamoDB

[pontuspalmenas]

In the v1 SDK, it was possible to update an item to add a value to a set, eg. string to a string set, by using the .setSS function of AttributeValue (aws/aws-sdk-go#1990).

I have tried using a slice directly with expression:

expr, err := expression.NewBuilder().
WithUpdate(
	expression.Add(expression.Name("my-string-set"), expression.Value([]string{"my-string"})).
).Build()

This gives the error ValidationException: Invalid UpdateExpression: Incorrect operand type for operator or function; operator: ADD, operand type: LIST, typeSet: ALLOWED_FOR_ADD_OPERAND

I have tried using the types.AttributeValueMemberSS:

ss := types.AttributeValueMemberSS{Value: []string{"my-string"}}
expr, err := expression.NewBuilder().
	WithUpdate(
		expression.Add(expression.Name("my-string-set"), expression.Value(ss))
	).Build()

This gives me the error ValidationException: The provided key element does not match the schema: errorString

I don't believe this is possible in v2. Please correct me if I'm wrong.