2.6 secondaries abort when replicated index spec contains a dollar-sign field

var replSet = new ReplSetTest({ name: 'mixedVersionIndexes', nodes: 3, nodeOptions: { smallfiles: '', binVersion: MongoRunner.versionIterator(['2.4', '2.6']) } });   replSet.startSet(); replSet.initiate(); replSet.awaitSecondaryNodes();   // make a 2.4 node the primary assert.soon(function() { var primary = replSet.getPrimary();   if (primary.getDB('admin').version().startsWith('2.4') === false) { try { primary.getDB('admin').runCommand({replSetStepDown: 60}); } catch(e) { jsTest.log(e); } return false; }   return true; }, 'failed to elect 2.4 node as primary', 60000, 30000);   // create an invalid 2.6 index on the 2.4 primary var coll = replSet.getPrimary().getDB('foo').bar; coll.ensureIndex({$foo:1});   replSet.awaitReplication();   jsTest.log('available nodes:\n' + tojson(replSet.liveNodes));