diff --git a/android/src/main/java/com/xiarui/zhiwen/UsbController.java b/android/src/main/java/com/xiarui/zhiwen/UsbController.java
index 12ada17..f79eefd 100644
--- a/android/src/main/java/com/xiarui/zhiwen/UsbController.java
+++ b/android/src/main/java/com/xiarui/zhiwen/UsbController.java
@@ -424,7 +424,11 @@ public class UsbController {
             new IPermissionListener() {
                 @Override
                 public void onPermissionDenied(UsbDevice d) {
-                    l("Permission denied on " + d.getDeviceId());
+                    if (d != null) {
+                        l("Permission denied on " + d.getDeviceId());
+                    } else {
+                        l("Permission denied (device unavailable)");
+                    }
                 }
             });