diff --git a/extmod/modos.c b/extmod/modos.c
index eda359b97b..7f1e31fba7 100644
--- a/extmod/modos.c
+++ b/extmod/modos.c
@@ -76,8 +76,9 @@
 static mp_obj_t mp_os_sync(void) {
     #if MICROPY_VFS_FAT
     for (mp_vfs_mount_t *vfs = MP_STATE_VM(vfs_mount_table); vfs != NULL; vfs = vfs->next) {
-        // this assumes that vfs->obj is fs_user_mount_t with block device functions
-        disk_ioctl(MP_OBJ_TO_PTR(vfs->obj), CTRL_SYNC, NULL);
+        if (mp_obj_is_type(vfs->obj, &mp_fat_vfs_type)) {
+            disk_ioctl(MP_OBJ_TO_PTR(vfs->obj), CTRL_SYNC, NULL);
+        }
     }
     #endif
     return mp_const_none;