diff --git a/doc/src_code/insensitive.cpp b/doc/src_code/insensitive.cpp
index 2575d72c..8cf29f1b 100644
--- a/doc/src_code/insensitive.cpp
+++ b/doc/src_code/insensitive.cpp
@@ -19,7 +19,7 @@
     };
 
     struct ihash
-        : std::unary_function<std::string, bool>
+        : std::unary_function<std::string, std::size_t>
     {
         std::size_t operator()(std::string const& x) const
         {