Invalida operation for task

[ArthurRM]

I have the following code:

final JobResponse createJobResponse = client.jobs().create(
            ImmutableMap.of(
                "import-pdf", new UrlImportRequest().setUrl("https://www.w3.org/WAI/ER/tests/xhtml/testfiles/resources/pdf/dummy.pdf"),
                "pdf-to-png", new ConvertFilesTaskRequest()
                    .setInput("import-pdf")
                    .setInputFormat("pdf")
                    .setOutputFormat("png")
                    .set("width", 300)
                    .set("height", 400),
                "export-png", new UrlExportRequest().setInput("pdf-to-png")
            )
        ).getBody();

But I get an error:

ErrorResponse(code=INVALID_DATA, message=Invalid operation for task export-png: EXPORT_URL, errors={tasks=[Invalid operation for task export-png: EXPORT_URL]})

Why?