Problem with 2dsphere index?

Question

MongoDB

I use the Loopback framework, the model has a gpsLocation field with the geopoint type, after I created a 2dsphere index for this field, I started getting an error like:
"Can't extract geo keys: { _id: ObjectId('59cb8c43b296d3a81f43ad3a'), gpsLocation: { lat : 41.931696, lng: 115.9889556 } } can't project geometry into spherical CRS: { lat: 41.931696, lng: 115.9889556 }"
But I can't figure out what the problem is and how to solve it, can anyone help?