diff --git a/vtkm/cont/Storage.h b/vtkm/cont/Storage.h
index 02f66955d..c72022448 100644
--- a/vtkm/cont/Storage.h
+++ b/vtkm/cont/Storage.h
@@ -72,7 +72,7 @@ namespace detail {
 // arguments.
 template<typename T>
 struct UndefinedArrayPortal {
-  BOOST_STATIC_ASSERT(sizeof(T) == -1);
+  BOOST_STATIC_ASSERT(sizeof(T) == static_cast<size_t>(-1));
 };
 
 } // namespace detail