diff --git a/pdftext/schema.py b/pdftext/schema.py
index 7954d5f..c6e8e95 100644
--- a/pdftext/schema.py
+++ b/pdftext/schema.py
@@ -7,6 +7,7 @@
 class Bbox:
     def __init__(self, bbox: List[float], ensure_nonzero_area=False):
         if ensure_nonzero_area:
+            bbox = list(bbox)
             bbox[2] = max(bbox[0], bbox[2] + 1)
             bbox[3] = max(bbox[1], bbox[3] + 1)
         self.bbox = bbox