diff --git a/include/pybind11/numpy.h b/include/pybind11/numpy.h
index 3227a12e..146dba43 100644
--- a/include/pybind11/numpy.h
+++ b/include/pybind11/numpy.h
@@ -340,11 +340,11 @@ public:
     template <typename... Ix> T *mutable_data(Ix... ix) { return &operator()(size_t(ix)...); }
 };
 
-template <typename T, size_t Dim>
+template <typename T, ssize_t Dim>
 struct type_caster<unchecked_reference<T, Dim>> {
     static_assert(Dim == 0 && Dim > 0 /* always fail */, "unchecked array proxy object is not castable");
 };
-template <typename T, size_t Dim>
+template <typename T, ssize_t Dim>
 struct type_caster<unchecked_mutable_reference<T, Dim>> : type_caster<unchecked_reference<T, Dim>> {};
 
 NAMESPACE_END(detail)