diff --git a/layers/box_utils.py b/layers/box_utils.py
index 84214947b..d71284c4e 100644
--- a/layers/box_utils.py
+++ b/layers/box_utils.py
@@ -22,8 +22,8 @@ def center_size(boxes):
     Return:
         boxes: (tensor) Converted xmin, ymin, xmax, ymax form of boxes.
     """
-    return torch.cat((boxes[:, 2:] + boxes[:, :2])/2,  # cx, cy
-                     boxes[:, 2:] - boxes[:, :2], 1)  # w, h
+    return torch.cat(((boxes[:, 2:] + boxes[:, :2])/2,  # cx, cy
+                     boxes[:, 2:] - boxes[:, :2]), 1)  # w, h
 
 
 def intersect(box_a, box_b):