diff --git a/src/twomartens/masterthesis/main.py b/src/twomartens/masterthesis/main.py
index 0f93e94..384ee6b 100644
--- a/src/twomartens/masterthesis/main.py
+++ b/src/twomartens/masterthesis/main.py
@@ -209,7 +209,7 @@ def _ssd_test(args: argparse.Namespace) -> None:
             _labels = pickle.load(file)
             # exclude padded label entries
             real_labels = np.nonzero(_labels[:, :, 0] != -1)
-            labels.extend(_labels[real_labels])
+            labels.extend(_labels[_labels[:, :, 0] != -1])
     # store labels for later use
     with open(label_file, "wb") as file:
         pickle.dump(labels, file)