Passing temporary s3 credentials to assembly

Has anyone successfully passed AWS temporary credentials for s3 to an assembly?

I have a template that reads & writes to s3. When I execute assembly (in an AWS lambda), I set the key & secret using a set of temporary credentials. What I get from the assembly is:

An error occurred (InvalidAccessKeyId) when calling the GetObject operation: The AWS Access Key Id you provided does not exist in our records.

This make me wonder if Transloadit will only access a statically generated credential pair rather than a temporary pair (I’m particularly suspicious because the associated token for the temporary credentials cannot be passed).

When I pass a static pair, it works.