diff --git a/main.lua b/main.lua
index cfb463d..0ff4e71 100644
--- a/main.lua
+++ b/main.lua
@@ -785,7 +785,6 @@ function Bookhoard:collectBookData()
         title = title,
         authors = authors,
         percentage = percentage,
-        epubcfi = progress,
         context_text = context_text,
         page = page,
         total_pages = total_pages,
@@ -796,6 +795,15 @@ function Bookhoard:collectBookData()
         },
     }
 
+    -- epubcfi (a CREngine xpointer) is only meaningful for reflowable (rolling)
+    -- documents. Paging docs (PDF/comics/DjVu) carry their position in
+    -- page/total_pages; a bare page number here would be a JSON number into a
+    -- server *string field and fail the bind. Reflowable EPUBs are always
+    -- rolling, so they keep sending the xpointer exactly as before.
+    if not self.ui.document.info.has_pages then
+        book_data.epubcfi = progress
+    end
+
     return book_data
 end