diff --git a/extmod/vfs.c b/extmod/vfs.c
index ae09b8afe8..5b8f3ccb0f 100644
--- a/extmod/vfs.c
+++ b/extmod/vfs.c
@@ -273,7 +273,7 @@ mp_obj_t mp_vfs_umount(mp_obj_t mnt_in) {
         mnt_str = mp_obj_str_get_data(mnt_in, &mnt_len);
     }
     for (mp_vfs_mount_t **vfsp = &MP_STATE_VM(vfs_mount_table); *vfsp != NULL; vfsp = &(*vfsp)->next) {
-        if ((mnt_str != NULL && !memcmp(mnt_str, (*vfsp)->str, mnt_len + 1)) || (*vfsp)->obj == mnt_in) {
+        if ((mnt_str != NULL && !memcmp(mnt_str, (*vfsp)->str, (((*vfsp)->len < mnt_len + 1) ? (*vfsp)->len : mnt_len + 1))) || (*vfsp)->obj == mnt_in) {
             vfs = *vfsp;
             *vfsp = (*vfsp)->next;
             break;