$sample aggregate not functioning correctly in MongoDB

i am trying to grab a random document from my collection and I keep receiving the error:

PlanExecutor error during aggregation :: caused by :: The argument to $size must be an array, but was of type: missing

obviously not trying to use the $size aggregate just the parameter with the $sample aggregate.

[{
 $project: {
  _id: '$_id',
  sender_id: '$sender_id',
  date_time: '$date_time',
  numberOfLikes: {
   $size: '$favorited_by'
  }
 }
}, {
 $match: {
  numberOfLikes: {
   $gte: 9
  }
 }
}, {
 $lookup: {
  from: 'UsedClessics',
  localField: '_id',
  foreignField: 'reply_id',
  as: 'UsedClessics'
 }
}, {
 $match: {
  'UsedClessics.type': {
   $ne: 'reply'
  }
 }
}, {
 $match: {
  sender_id: {
   $ne: 'system'
  }
 }
}, {
 $match: {
  sender_id: {
   $ne: '848846'
  }
 }
}, {
 $sample: {
  size: 1
 }
}]