Currently if you mistakenly send a non-string as the key field no error is generated. Both the command and shell helper should be checking this.

> db.dbref.find({"ref.$ref":"extra"})
{ "_id" : 1, "ref" : DBRef("extra", 2) }
> db.dbref.distinct({"ref.$ref":"extra"}) // should generate an error
[ ]

The command turns this into the key spec {"":1} because of "cmdObj["key"].valuestrsafe();" returning "" for non strings.

In both places we just need to make sure the arg is a string, or error.

Likewise the query option should also be validated since anything other than embedded doc will result in the value being ignored and replaced by the empty query/doc.