diff --git a/include/boost/python/with_custodian_and_ward.hpp b/include/boost/python/with_custodian_and_ward.hpp
index 2b829138..7c49baf0 100644
--- a/include/boost/python/with_custodian_and_ward.hpp
+++ b/include/boost/python/with_custodian_and_ward.hpp
@@ -84,7 +84,7 @@ struct with_custodian_and_ward_postcall : BasePolicy_
     template <class ArgumentPackage>
     static PyObject* postcall(ArgumentPackage const& args_, PyObject* result)
     {
-        unsigned arity_ = detail::arity(args_);
+        std::size_t arity_ = detail::arity(args_);
         if ( custodian > arity_ || ward > arity_ )
         {
             PyErr_SetString(