diff --git a/src/calibre/devices/prst1/driver.py b/src/calibre/devices/prst1/driver.py
index 44c93af4cc..eeb73da182 100644
--- a/src/calibre/devices/prst1/driver.py
+++ b/src/calibre/devices/prst1/driver.py
@@ -443,7 +443,7 @@ class PRST1(USBMS):
 
     def upload_book_cover(self, connection, book, source_id):
         debug_print('PRST1: Uploading/Refreshing Cover for ' + book.title)
-        if not book.thumbnail and book.thumbnail[-1]:
+        if not book.thumbnail or not book.thumbnail[-1]:
             return
         cursor = connection.cursor()