diff --git a/lib/src/tree_printer.dart b/lib/src/tree_printer.dart
index 0d18400..338b7e0 100644
--- a/lib/src/tree_printer.dart
+++ b/lib/src/tree_printer.dart
@@ -24,7 +24,7 @@
   @override
   void visitTree(StyleSheet tree) => visitStylesheet(tree);
 
-  void heading(String heading, node) {
+  void heading(String heading, TreeNode node) {
     if (useSpan) {
       output.heading(heading, node.span);
     } else {