diff --git a/include/gsl/span_ext b/include/gsl/span_ext
index d86f736..5f54e81 100644
--- a/include/gsl/span_ext
+++ b/include/gsl/span_ext
@@ -41,7 +41,7 @@ namespace gsl
 {
 
 // [span.views.constants], constants
-constexpr const std::size_t dynamic_extent = narrow_cast<std::size_t>(-1);
+inline constexpr const std::size_t dynamic_extent = narrow_cast<std::size_t>(-1);
 
 template <class ElementType, std::size_t Extent = dynamic_extent>
 class span;