diff --git a/src/twomartens/masterthesis/ssd.py b/src/twomartens/masterthesis/ssd.py
index 566ec3e..8bc7ac2 100644
--- a/src/twomartens/masterthesis/ssd.py
+++ b/src/twomartens/masterthesis/ssd.py
@@ -272,7 +272,7 @@ def _get_observations(detections: Sequence[Sequence[np.ndarray]]) -> List[List[n
         detections_image = np.asarray(detections[i])
         class_ids = np.argmax(detections_image[:, :-12],
                               axis=-1)
-        detections_image = detections_image[class_ids != 0]
+        detections_image = detections_image[np.nonzero(class_ids[:])]
         print(detections_image.shape)
         overlaps = bounding_box_utils.iou(detections_image[:, -12:-8],
                                           detections_image[:, -12:-8],