Found an index like {fieldName: 0}

I just did a mongodump and mongorestore and got an error:

{{code}
Failed: jerome5.Unit: error creating indexes for jerome5.Unit: createIndex error: Values in the index key pattern cannot be 0.
code

I checked on the source collection and found an index like:

code
{
"key":

,
"name": "f_0",
"ns": "jerome5.Unit",
"v": 1
},
code

I'm not sure how long this has been there, but I suppose it doesn't actually do anything on the source database? Maybe it is assumed to be 1 if it's not -1 ?