diff --git a/u01/node.cpp b/u01/node.cpp
index dd3690b..3f0e0c0 100644
--- a/u01/node.cpp
+++ b/u01/node.cpp
@@ -80,7 +80,7 @@ std::string node::print_iterative() const {
       // Complex iteration (not default vector iteration is necessary,
       // to achieve the same output as the recursive approach.
       // Otherwise, the order of the children is reversed (due to LIFO).
-      for (unsigned int i = n->get_nr_children(); i > 0; i--) {
+      for (std::size_t i = n->get_nr_children(); i > 0; i--) {
         stack.push(std::make_pair(n->get_child(i - 1), depth + 1));
       }
     } else {