diff --git a/src/calibre/devices/usbms/device.py b/src/calibre/devices/usbms/device.py
index 6d86a3dc71..9a44289a4f 100644
--- a/src/calibre/devices/usbms/device.py
+++ b/src/calibre/devices/usbms/device.py
@@ -42,7 +42,7 @@ class USBDevice:
         if iswindows:
             # Getting this information requires communicating with the device
             # we only do that in the can_handle_windows() method, if needed.
-            self.manufacturer = self.serial = self.product = ''
+            self.manufacturer = self.serial = self.product = None
         else:
             self.manufacturer = dev[3]
             self.product = dev[4]