diff --git a/brother_ql/backends/pyusb.py b/brother_ql/backends/pyusb.py
index 1a843a8..2faf845 100755
--- a/brother_ql/backends/pyusb.py
+++ b/brother_ql/backends/pyusb.py
@@ -98,8 +98,8 @@ class BrotherQLBackendPyUSB(BrotherQLBackendGeneric):
         self.dev.set_configuration()
 
         cfg = self.dev.get_active_configuration()
-        intf = cfg[(0,0)] # (bInterfaceNumber, bAlternateSetting)
-        assert intf.bInterfaceClass == 7 # 'Printer' not 'Vendor Specific' or the likes...
+        intf = usb.util.find_descriptor(cfg, bInterfaceClass=7)
+        assert intf is not None
 
         ep_match_in  = lambda e: usb.util.endpoint_direction(e.bEndpointAddress) == usb.util.ENDPOINT_IN
         ep_match_out = lambda e: usb.util.endpoint_direction(e.bEndpointAddress) == usb.util.ENDPOINT_OUT