Mongoose Schema.pre insertMany middleware

I'm using mongoose with nodeJs and I'm trying to implement a middleware called when using insertMany . In mongoose documentation there are no example at all, but they confirm that insertMany model function triggers the following middleware: insertMany().

I have a basic User schema and I need insertMany hooks to hash password with bcrypt:

UserSchema.pre('insertMany', async function (err, docs, next) {

  try{
    docs.map(async function (doc, index) {
      // async hash password
      doc.password = await User.hashPassword(doc.password);
    });

  } catch (error) {
    console.log(error);
  }

  next();
});

I'm not posting all schema because all the rest works, including password hashing and 'save' hooks. I cannot understand why I am getting this error when I try to insert many users:

next is not a function

Usually I call next() to proceed to next middleware, but in this case it doesn't work. If I remove next(), code won't go on to next middleware and in both cases users are not inserted.

Can someone help me?