diff --git a/include/boost/parameter/aux_/arg_list.hpp b/include/boost/parameter/aux_/arg_list.hpp
index a031e3e..d70205f 100755
--- a/include/boost/parameter/aux_/arg_list.hpp
+++ b/include/boost/parameter/aux_/arg_list.hpp
@@ -11,6 +11,7 @@
 #include <boost/parameter/aux_/default.hpp>
 #include <boost/parameter/aux_/parameter_requirements.hpp>
 #include <boost/parameter/aux_/yesno.hpp>
+#include <boost/parameter/aux_/maybe.hpp>
 #include <boost/parameter/config.hpp>
 
 #include <boost/mpl/apply.hpp>
@@ -158,10 +159,6 @@ no_tag operator*(empty_arg_list, KW*);
 template <class KW, class T>
 struct tagged_argument;
 
-// Forward declaration for arg_list::operator[], with
-// IS_XXX helper
-struct maybe_base;
-
 template <class T>
 struct is_maybe
   : is_base_and_derived<maybe_base, T>