diff --git a/src/TileGroup.jsx b/src/TileGroup.jsx
index ee60004e..824e2077 100644
--- a/src/TileGroup.jsx
+++ b/src/TileGroup.jsx
@@ -54,8 +54,8 @@ TileGroup.propTypes = {
   ...tileGroupProps,
   activeStartDate: PropTypes.instanceOf(Date),
   count: PropTypes.number,
-  dateType: PropTypes.string,
   dateTransform: PropTypes.func.isRequired,
+  dateType: PropTypes.string,
   offset: PropTypes.number,
   step: PropTypes.number,
   tile: PropTypes.func.isRequired,