diff --git a/src/relations/Bounds.js b/src/relations/Bounds.js
index 9f7f235..3ebae4b 100644
--- a/src/relations/Bounds.js
+++ b/src/relations/Bounds.js
@@ -93,15 +93,19 @@ const mergeBounds = clientBounds => {
     };
 
     const extend = function(a, b) {
-      a.x = Math.min(a.x, b.x);
-      a.left = Math.min(a.left, b.left);
-      a.width = a.width + b.width;
-      a.right = Math.max(a.right + b.right);
+      const x = Math.min(a.x, b.x);
+      const left = Math.min(a.left, b.left);
+      const y = Math.max(a.y, b.y);
+      const right = Math.max(a.right, b.right);
+      const width = a.width + b.width;
+
+      const { bottom, height, top } = a;
+      return { bottom, height, left, right, top, width, x, y };
     };
 
     if (previous) {
       if (isConsecutive(previous, bbox))
-        extend(previous, bbox);
+        merged[merged.length - 1] = extend(previous, bbox);
       else
         merged.push(bbox);
     } else {