diff --git a/src/xfns.c b/src/xfns.c
index 2b84c6d0376..4f23af432c8 100644
--- a/src/xfns.c
+++ b/src/xfns.c
@@ -2805,7 +2805,7 @@ validate_x_resource_name ()
 	     || (c >= 'A' && c <= 'Z')
 	     || (c >= '0' && c <= '9')
 	     || c == '-' || c == '_'))
-	SREF (new, i) = '_';
+	SSET (new, i, '_');
     }
 }