Testing PdfDocument with RobolectricTestRunner/AndroidJUnit4: document is closed

I am trying to test with Robolectric or AndroidJUnit4 some pdf generation in my android application (using android.graphics.pdf.PdfDocument).

@Test
fun testPdfDocument() {
    val doc = PdfDocument()
    val builder = PdfDocument.PageInfo.Builder(20, 10, 1)
    val pdf = builder.create()
    assertEquals(0, doc.pages.size)
    val page = doc.startPage(pdf) // java.lang.IllegalStateException: document is closed!
    doc.finishPage(page)
    assertEquals(1, doc.pages.size)
}

The code above crashes, with java.lang.IllegalStateException: document is closed!. Any idea why?