diff --git a/src/twomartens/masterthesis/debug.py b/src/twomartens/masterthesis/debug.py
index c7ca527..5b6bbfc 100644
--- a/src/twomartens/masterthesis/debug.py
+++ b/src/twomartens/masterthesis/debug.py
@@ -83,7 +83,7 @@ def save_ssd_train_images(images: np.ndarray, labels: np.ndarray,
                 xmax = instance[5]
                 ymax = instance[6]
             else:
-                instance = np.asarray(instance)
+                instance = np.copy(instance)
                 class_id = np.argmax(instance[:-12], axis=0)
                 instance[-12:-8] *= instance[-4:]  # multiply with variances
                 instance[[-11, -9]] *= np.expand_dims(instance[-5] - instance[-7], axis=-1)