diff --git a/include/uuid.h b/include/uuid.h
index b75c9b5..c91f5a0 100644
--- a/include/uuid.h
+++ b/include/uuid.h
@@ -16,7 +16,12 @@
 #include <chrono>
 #include <numeric>
 #include <atomic>
+
+#ifdef UUID_USING_CXX20_SPAN
 #include <span>
+#else
+#include <gsl/gsl>
+#endif
 
 #ifdef _WIN32
 
@@ -950,4 +955,4 @@ namespace std
    };
 }
 
-#endif /* STDUUID_H */
\ No newline at end of file
+#endif /* STDUUID_H */