diff --git a/packages/flutter/lib/src/material/expansion_tile.dart b/packages/flutter/lib/src/material/expansion_tile.dart
index e288c2d0dd7..b98cd772161 100644
--- a/packages/flutter/lib/src/material/expansion_tile.dart
+++ b/packages/flutter/lib/src/material/expansion_tile.dart
@@ -112,7 +112,7 @@ class _ExpansionTileState extends State<ExpansionTile> with SingleTickerProvider
       if (_isExpanded)
         _controller.forward();
       else
-        _controller.reverse().then((Null value) {
+        _controller.reverse().then<Null>((Null value) {
           setState(() {
             // Rebuild without widget.children.
           });