diff --git a/src/grits.py b/src/grits.py
index c9d48280e..c405e3352 100644
--- a/src/grits.py
+++ b/src/grits.py
@@ -226,8 +226,8 @@ def iou(bbox1, bbox2):
     """
     Compute the intersection-over-union of two bounding boxes.
     """
-    intersection = Rect(bbox1).intersect(bbox2)
-    union = Rect(bbox1).include_rect(bbox2)
+    intersection = Rect(list(bbox1)).intersect(list(bbox2))
+    union = Rect(list(bbox1)).include_rect(list(bbox2))
     
     union_area = union.get_area()
     if union_area > 0: