diff --git a/src/calibre/gui2/preferences/device_user_defined.py b/src/calibre/gui2/preferences/device_user_defined.py
index ae963f61c1..40413593d8 100644
--- a/src/calibre/gui2/preferences/device_user_defined.py
+++ b/src/calibre/gui2/preferences/device_user_defined.py
@@ -64,6 +64,7 @@ class UserDefinedDevice(QDialog):
             res = ''
             if len(new_devices) == 1:
                 def fmtid(x):
+                    x = x or 0
                     if isinstance(x, numbers.Integral):
                         x = hex(x)
                     if not x.startswith('0x'):