diff --git a/include/cxxopts.hpp b/include/cxxopts.hpp
index 0d37e33..e08f46a 100644
--- a/include/cxxopts.hpp
+++ b/include/cxxopts.hpp
@@ -858,6 +858,7 @@ namespace cxxopts
   };
 
   // A helper function for setting required arguments
+  inline
   void
   check_required
   (