diff --git a/src/twomartens/masterthesis/main.py b/src/twomartens/masterthesis/main.py
index 9c8e13f..0f93e94 100644
--- a/src/twomartens/masterthesis/main.py
+++ b/src/twomartens/masterthesis/main.py
@@ -182,7 +182,8 @@ def _test(args: argparse.Namespace) -> None:
 def _ssd_test(args: argparse.Namespace) -> None:
     import glob
     import pickle
-    
+
+    import numpy as np
     import tensorflow as tf
 
     from twomartens.masterthesis import evaluate
@@ -207,7 +208,7 @@ def _ssd_test(args: argparse.Namespace) -> None:
             # get labels per batch
             _labels = pickle.load(file)
             # exclude padded label entries
-            real_labels = _labels[:, :, 0] != -1
+            real_labels = np.nonzero(_labels[:, :, 0] != -1)
             labels.extend(_labels[real_labels])
     # store labels for later use
     with open(label_file, "wb") as file: