This looks like database corruption. There appears to be a record in the units collection with an empty "tools" attribute. A dump of the units collection should show the affected unit records. These should match the units in error.
mongo would need to be updated to repair those records to match other units.
This looks like database corruption. There appears to be a record in the units collection with an empty "tools" attribute. A dump of the units collection should show the affected unit records. These should match the units in error.
mongo would need to be updated to repair those records to match other units.