You look to have the correct syntax - have you built this transform or are you just looking at the preview?
If you run the preview, your set will be empty because a PreviewIncrementalTransformOutput is used instead of an IncrementalTransformOutput , which will always return an empty dataframe if you use previous mode. A build will properly execute the incremental transforms logic.
I confirmed this by running a preview and looking in the debug panel to see the type of the output.
