Why does NotebookLM say "Source is empty"?

"Source is empty" means NotebookLM fetched the page but found no readable text in it. The most common causes:

JavaScript-rendered pages: NotebookLM reads raw HTML. If a page's content only appears after scripts run (single-page apps, dynamic dashboards), the source arrives empty.

Image-only PDFs: Scanned documents with no text layer cannot be read. Run OCR first using Google Drive, Adobe Acrobat, or a free online OCR tool, then re-upload.

YouTube without captions: Videos with no auto-generated or manual captions have no transcript to import.

The fix in all cases: generate or extract the text yourself, then paste it as a text source manually.