If you run createIndexes with an index spec that includes a blank namespace, you get a 'namespace mismatch' error. The mismatch is technically true, but it'd be more user-friendly to detect blank namespaces and have a more specific error.

> var spec = {key: {'foo': 1}, name: 'blank', ns: ''};
> db.getSiblingDB('repro').foo.runCommand('createIndexes', {indexes: [spec]});
{
	"spec" : {
		"key" : {
			"foo" : 1
		},
		"name" : "blank",
		"ns" : ""
	},
	"ok" : 0,
	"errmsg" : "namespace mismatch"
}

Version: b605f98c68a03fd0e65be72f9a22af4f107dee6b