diff --git a/ports/nrf/main.c b/ports/nrf/main.c
index 1013005e19..bb3bcec36d 100644
--- a/ports/nrf/main.c
+++ b/ports/nrf/main.c
@@ -312,11 +312,6 @@ mp_lexer_t *mp_lexer_new_from_file(qstr filename) {
 mp_import_stat_t mp_import_stat(const char *path) {
     return MP_IMPORT_STAT_NO_EXIST;
 }
-
-mp_obj_t mp_builtin_open(size_t n_args, const mp_obj_t *args, mp_map_t *kwargs) {
-    mp_raise_OSError(MP_EPERM);
-}
-MP_DEFINE_CONST_FUN_OBJ_KW(mp_builtin_open_obj, 1, mp_builtin_open);
 #endif
 #endif
 
diff --git a/ports/nrf/mpconfigport.h b/ports/nrf/mpconfigport.h
index 80e37bc4f7..cad839d0e9 100644
--- a/ports/nrf/mpconfigport.h
+++ b/ports/nrf/mpconfigport.h
@@ -353,10 +353,11 @@ long unsigned int rng_generate_random_word(void);
 #include "boardmodules.h"
 #endif // BOARD_SPECIFIC_MODULES
 
-// extra built in names to add to the global namespace
+#if MICROPY_MBFS
+// The builtins.open function must be explicitly added when using the micro:bit filesystem.
 #define MICROPY_PORT_BUILTINS \
-    { MP_ROM_QSTR(MP_QSTR_help), MP_ROM_PTR(&mp_builtin_help_obj) }, \
-    { MP_ROM_QSTR(MP_QSTR_open), MP_ROM_PTR(&mp_builtin_open_obj) }, \
+    { MP_ROM_QSTR(MP_QSTR_open), MP_ROM_PTR(&mp_builtin_open_obj) },
+#endif
 
 // extra constants
 #define MICROPY_PORT_CONSTANTS \