diff --git a/include/boost/python/detail/caller.hpp b/include/boost/python/detail/caller.hpp
index c572d35a..1bd30bfb 100644
--- a/include/boost/python/detail/caller.hpp
+++ b/include/boost/python/detail/caller.hpp
@@ -48,7 +48,7 @@ inline PyObject* get(mpl::int_<N>, PyObject* const& args_)
     return PyTuple_GET_ITEM(args_,N);
 }
 
-inline unsigned arity(PyObject* const& args_)
+inline Py_ssize_t arity(PyObject* const& args_)
 {
     return PyTuple_GET_SIZE(args_);
 }