diff --git a/include/gsl/span b/include/gsl/span
index 52af1f7..b060113 100644
--- a/include/gsl/span
+++ b/include/gsl/span
@@ -71,7 +71,7 @@ namespace gsl
 {
 
 // [views.constants], constants
-constexpr const std::size_t dynamic_extent = -1;
+constexpr const std::size_t dynamic_extent = static_cast<std::size_t>(-1);
 
 template <class ElementType, std::size_t Extent = dynamic_extent>
 class span;