diff --git a/src/config.js b/src/config.js
index 0e40b6e..d64a783 100644
--- a/src/config.js
+++ b/src/config.js
@@ -106,6 +106,7 @@ c3_chart_internal_fn.getDefaultConfig = function () {
         axis_y_max: undefined,
         axis_y_min: undefined,
         axis_y_center: undefined,
+        axis_y_inner: undefined,
         axis_y_label: {},
         axis_y_tick_format: undefined,
         axis_y_tick_outer: true,
@@ -119,6 +120,7 @@ c3_chart_internal_fn.getDefaultConfig = function () {
         axis_y2_max: undefined,
         axis_y2_min: undefined,
         axis_y2_center: undefined,
+        axis_y2_inner: undefined,
         axis_y2_label: {},
         axis_y2_tick_format: undefined,
         axis_y2_tick_outer: true,