diff --git a/python/port/mod/matplotlib/pyplot/modpyplot.cpp b/python/port/mod/matplotlib/pyplot/modpyplot.cpp
index e8bfff604..4381c2520 100644
--- a/python/port/mod/matplotlib/pyplot/modpyplot.cpp
+++ b/python/port/mod/matplotlib/pyplot/modpyplot.cpp
@@ -114,7 +114,11 @@ mp_obj_t modpyplot_axis(size_t n_args, const mp_obj_t *args) {
       } else {
         mp_raise_ValueError("Unrecognized string given to axis; try 'on', 'off' or 'auto'");
       }
-#warning Use mp_obj_is_bool when upgrading uPy
+#if MICROPY_OBJ_IMMEDIATE_OBJS
+/* This couldn't be done at the time of writing because mp_obj_is_bool didn't
+ * exist just yet. */
+#error Use mp_obj_is_bool instead of mp_obj_is_type
+#endif
     } else if (mp_obj_is_type(arg, &mp_type_bool)) {
       sPlotStore->setAxesRequested(mp_obj_is_true(arg));
     } else if (mp_obj_is_type(arg, &mp_type_tuple) || mp_obj_is_type(arg, &mp_type_list)) {