MongoDB: Cannot use complex condition in subdocument

I would like to count all rooms for the particular user with particular roles.

Here is my schema:

const RoomSchema = new Schema({
  ...
  moderator: String,

  // Subdocument with two fields *email* and *role*
  participants: {
    type: [{
      _id: false,
      email: {
        type: String,
        index: {
          unique: true,
        },
      },
      role: String,
    }],
    default: [],
  },
  ...
})

Here is the query:

  let userEmail = "xxx"

  const result = await RoomModel
    .count({
      $or: [
        { moderator: userEmail },
        {
          participants: {
            email: userEmail,
            // role: "reader", works fine
            // role: { $in: ["reader"] }, ERROR
          }
        },
      ]
    })
    .exec()

Here is the error:

CastError: Cast to string failed for value "{ '$in': [ 'reader' ] }" (type Object) at path "role" for model "Room"

Thank you in advance.