diff --git a/packages/netlify-cms-core/src/mediaLibrary.ts b/packages/netlify-cms-core/src/mediaLibrary.ts
index f85d782a..940e6994 100644
--- a/packages/netlify-cms-core/src/mediaLibrary.ts
+++ b/packages/netlify-cms-core/src/mediaLibrary.ts
@@ -5,6 +5,7 @@
 import { once } from 'lodash';
 import { getMediaLibrary } from './lib/registry';
 import store from './redux';
+import { configFailed } from './actions/config';
 import { createMediaLibrary, insertMedia } from './actions/mediaLibrary';
 import { MediaLibraryInstance } from './types/redux';
 
@@ -18,10 +19,17 @@ interface MediaLibrary {
 }
 
 const initializeMediaLibrary = once(async function initializeMediaLibrary(name, options) {
-  const lib = (getMediaLibrary(name) as unknown) as MediaLibrary;
-  const handleInsert = (url: string) => store.dispatch(insertMedia(url, undefined));
-  const instance = await lib.init({ options, handleInsert });
-  store.dispatch(createMediaLibrary(instance));
+  const lib = (getMediaLibrary(name) as unknown) as MediaLibrary | undefined;
+  if (!lib) {
+    const err = new Error(
+      `Missing external media library '${name}'. Please use 'registerMediaLibrary' to register it.`,
+    );
+    store.dispatch(configFailed(err));
+  } else {
+    const handleInsert = (url: string) => store.dispatch(insertMedia(url, undefined));
+    const instance = await lib.init({ options, handleInsert });
+    store.dispatch(createMediaLibrary(instance));
+  }
 });
 
 store.subscribe(() => {
diff --git a/packages/netlify-cms-core/src/reducers/config.ts b/packages/netlify-cms-core/src/reducers/config.ts
index f462c78b..6e031be4 100644
--- a/packages/netlify-cms-core/src/reducers/config.ts
+++ b/packages/netlify-cms-core/src/reducers/config.ts
@@ -3,7 +3,9 @@ import { CONFIG_REQUEST, CONFIG_SUCCESS, CONFIG_FAILURE, CONFIG_MERGE } from '..
 import { Config, ConfigAction } from '../types/redux';
 import { EDITORIAL_WORKFLOW } from '../constants/publishModes';
 
-const config = (state = Map({ isFetching: true }), action: ConfigAction) => {
+const defaultState: Map<string, boolean | string> = Map({ isFetching: true });
+
+const config = (state = defaultState, action: ConfigAction) => {
   switch (action.type) {
     case CONFIG_MERGE:
       return state.mergeDeep(action.payload);
@@ -17,7 +19,10 @@ const config = (state = Map({ isFetching: true }), action: ConfigAction) => {
        */
       return action.payload.delete('isFetching');
     case CONFIG_FAILURE:
-      return Map({ error: action.payload.toString() });
+      return state.withMutations(s => {
+        s.delete('isFetching');
+        s.set('error', action.payload.toString());
+      });
     default:
       return state;
   }
diff --git a/packages/netlify-cms-core/src/types/redux.ts b/packages/netlify-cms-core/src/types/redux.ts
index c862fa66..10b60599 100644
--- a/packages/netlify-cms-core/src/types/redux.ts
+++ b/packages/netlify-cms-core/src/types/redux.ts
@@ -41,6 +41,7 @@ export type Config = StaticallyTypedRecord<{
   site_id?: string;
   site_url?: string;
   show_preview_links?: boolean;
+  isFetching?: boolean;
 }>;
 
 type PagesObject = {