message: In-memory keys need an object
diff: @@ -223,7 +223,7 @@ class MatrixClientPeg {
  if (SettingsStore.isFeatureEnabled("feature_cross_signing")) {
  // TODO: Cross-signing keys are temporarily in memory only. A
  // separate task in the cross-signing project will build from here.
  - const keys = [];
  + const keys = {};
  opts.cryptoCallbacks = {
  getCrossSigningKey: k => keys[k],
  saveCrossSigningKeys: newKeys => Object.assign(keys, newKeys),
  