diff --git a/src/calibre/devices/prst1/driver.py b/src/calibre/devices/prst1/driver.py
index a18c2f0eda..ea0f21871f 100644
--- a/src/calibre/devices/prst1/driver.py
+++ b/src/calibre/devices/prst1/driver.py
@@ -505,6 +505,8 @@ class PRST1(USBMS):
         debug_print('PRST1: Uploading/Refreshing Cover for ' + book.title)
         if (not book.thumbnail or isinstance(book.thumbnail, ImageWrapper) or
                 not book.thumbnail[-1]):
+            # If the thumbnail is an ImageWrapper instance, it refers to a book
+            # not in the calibre library
             return
         cursor = connection.cursor()