diff --git a/hw/usb/bus.c b/hw/usb/bus.c
index 6fffab7bfa..9a74dc9560 100644
--- a/hw/usb/bus.c
+++ b/hw/usb/bus.c
@@ -500,6 +500,10 @@ static void usb_mask_to_str(char *dest, size_t size,
                             speeds[i].name);
         }
     }
+
+    if (pos == 0) {
+        snprintf(dest, size, "unknown");
+    }
 }
 
 void usb_check_attach(USBDevice *dev, Error **errp)