$avg returns no value in MongoDb query - mongodb

Question

I'm working with NodeJs and MongoDb on Raspbian. I'm using monk as a driver.
I have documents in the database that look like this:
var data = {
ga : id, // Group address format (string)
val : val, // raw value (float)
ts : now // JavaScript time stamp
}
I'm using this code to retrieve grouped and aggregated data:
const dbRawColl = req.db.get('raw'); // this is the collection
var project = {
$project:{
_id: 0,
d: { $add: [ new Date(0), "$ts" ] },
v: "$val"
}
};
// filter by (this works)
var match = { $match: { 'ga': '0/7/71' } };
// group results
var group = {
$group: {
_id: { day: { $dayOfMonth: '$d' } },
min: { $min: '$v' },
max: { $max: '$v' },
avg: { $avg: '$v' },
count: { $sum: 1 }
}
};
dbRawColl.aggregate([ match, project, group ], function(e, docs) {
// send "docs" here
}
The query works, I see a number of grouped results per day. The answer I get looks like this
[{ "_id": { "day": 20 },
"min": "16.38",
"max": "29.64",
"avg": 0,
"count": 2687},
... more results omitted for clarity
]
The question is: why does "min" and "max" returns proper results while "$avg" is always 0. What prevents "$avg" from doing it's job. It's hardly exactly 0 because of roughly 50000 samples in the DB with a huge range of values (from 10 to 30).
Side question: Why does "min" and "max" returns strings as a result? The value itself appears valid to me looking at the samples.
OS, Node, Express, MongoDb are all current version as of 8/2016.
Edit to answer question, this is a typical document:
{
ga : "0/7/71", // Group address format (string)
val : "24.45", // raw value (float)
ts : 12345678909 // JavaScript time stamp
}
The value is obviously a string. Some values are actual strings, so converting all to number will not work. But for the fields I ask for with my $match it's guaranteed that the values are convertable to Number.