diff --git a/src/calibre/devices/interface.py b/src/calibre/devices/interface.py
index d9b52ad9a4..15c9e9c55f 100644
--- a/src/calibre/devices/interface.py
+++ b/src/calibre/devices/interface.py
@@ -256,6 +256,8 @@ class DevicePlugin(Plugin):
 
     def set_progress_reporter(self, report_progress):
         '''
+        Set a function to report progress information.
+
         :param report_progress: Function that is called with a % progress
                                 (number between 0 and 100) for various tasks
                                 If it is called with -1 that means that the
diff --git a/src/calibre/devices/prst1/driver.py b/src/calibre/devices/prst1/driver.py
index 737371e245..a18c2f0eda 100644
--- a/src/calibre/devices/prst1/driver.py
+++ b/src/calibre/devices/prst1/driver.py
@@ -503,7 +503,8 @@ 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 or not book.thumbnail[-1]:
+        if (not book.thumbnail or isinstance(book.thumbnail, ImageWrapper) or
+                not book.thumbnail[-1]):
             return
         cursor = connection.cursor()