diff --git a/compute_metrics_reloaded.py b/compute_metrics_reloaded.py
index 7a71b50..414bd15 100644
--- a/compute_metrics_reloaded.py
+++ b/compute_metrics_reloaded.py
@@ -161,7 +161,7 @@ def compute_metrics_single_subject(prediction, reference, metrics, ref_map, pred
         unique_labels = np.unique(np.concatenate((unique_labels_reference, unique_labels_prediction)))
     else:
         # Get the unique labels that are present in the reference OR prediction images
-        unique_labels = np.unique(np.concatenate(list(ref_map.keys()), list(pred_map.keys())))
+        unique_labels = np.unique(np.concatenate((list(ref_map.keys()), list(pred_map.keys()))))
 
     # append entry into the output_list to store the metrics for the current subject
     metrics_dict = {'reference': reference, 'prediction': prediction}