diff --git a/include/gsl/span b/include/gsl/span
index f0f7f1f..fae1522 100644
--- a/include/gsl/span
+++ b/include/gsl/span
@@ -544,7 +544,7 @@ public:
 #endif // _MSC_VER
 
 private:
-    static bool CheckRange(index_type idx, index_type size) noexcept
+    static constexpr bool CheckRange(index_type idx, index_type size) noexcept
     {
         // Optimization:
         //