diff --git a/src/manager.c b/src/manager.c
index 17e58b2..423cbfa 100644
--- a/src/manager.c
+++ b/src/manager.c
@@ -121,7 +121,7 @@ static gboolean modem_reset_done(struct EG25Manager* manager)
 
 void modem_reset(struct EG25Manager *manager)
 {
-    int fd, ret, len = strlen(manager->modem_usb_id);
+    int fd, ret, len;
 
     if (manager->reset_timer)
         return;
@@ -131,6 +131,13 @@ void modem_reset(struct EG25Manager *manager)
         manager->suspend_timer = 0;
     }
 
+    if (!manager->modem_usb_id) {
+        g_warning("Unknown modem USB ID");
+        goto error;
+    }
+
+    len = strlen(manager->modem_usb_id);
+
     manager->modem_state = EG25_STATE_RESETTING;
 
     fd = open("/sys/bus/usb/drivers/usb/unbind", O_WRONLY);