diff --git a/include/nana/gui/widgets/detail/tree_cont.hpp b/include/nana/gui/widgets/detail/tree_cont.hpp
index 2490ab95..7dab9d4d 100644
--- a/include/nana/gui/widgets/detail/tree_cont.hpp
+++ b/include/nana/gui/widgets/detail/tree_cont.hpp
@@ -357,12 +357,12 @@ namespace detail
 			}
 
 			template<typename PredAllowChild>
-			unsigned distance_if(const node_type * node, PredAllowChild pac) const
+			std::size_t distance_if(const node_type * node, PredAllowChild pac) const
 			{
 				if(nullptr == node)	return 0;
 				const node_type * iterator = root_.child;
 
-				unsigned off = 0;
+				std::size_t off = 0;
 				std::stack<const node_type* > stack;
 
 				while(iterator && iterator != node)