message: null is expected so return null rather than undefined
diff: @@ -19,7 +19,7 @@ export class DocumentStorageService implements api.IDocumentStorageService {
  if (!requestVerion) {
  const versions = await this.getVersions(this.id, 1);
  if (versions.length === 0) {
  - return Promise.resolve<api.ISnapshotTree>(undefined);
  + return Promise.resolve<api.ISnapshotTree>(null);
  }
  requestVerion = versions[0];
  }
  