diff --git a/ext/bg/js/anki-note-builder.js b/ext/bg/js/anki-note-builder.js
index 8a707006..700d8237 100644
--- a/ext/bg/js/anki-note-builder.js
+++ b/ext/bg/js/anki-note-builder.js
@@ -16,7 +16,8 @@
  */
 
 class AnkiNoteBuilder {
-    constructor({audioSystem, renderTemplate}) {
+    constructor({anki, audioSystem, renderTemplate}) {
+        this._anki = anki;
         this._audioSystem = audioSystem;
         this._renderTemplate = renderTemplate;
     }
@@ -101,7 +102,7 @@ class AnkiNoteBuilder {
         }
     }
 
-    async injectScreenshot(definition, fields, screenshot, anki) {
+    async injectScreenshot(definition, fields, screenshot) {
         if (!this._containsMarker(fields, 'screenshot')) { return; }
 
         const now = new Date(Date.now());
@@ -109,7 +110,7 @@ class AnkiNoteBuilder {
         const data = screenshot.dataUrl.replace(/^data:[\w\W]*?,/, '');
 
         try {
-            await anki.storeMediaFile(filename, data);
+            await this._anki.storeMediaFile(filename, data);
         } catch (e) {
             return;
         }
diff --git a/ext/bg/js/anki.js b/ext/bg/js/anki.js
index c7f7c0cc..38823431 100644
--- a/ext/bg/js/anki.js
+++ b/ext/bg/js/anki.js
@@ -19,122 +19,119 @@
  * requestJson
  */
 
-/*
- * AnkiConnect
- */
-
 class AnkiConnect {
     constructor(server) {
-        this.server = server;
-        this.localVersion = 2;
-        this.remoteVersion = 0;
+        this._enabled = false;
+        this._server = server;
+        this._localVersion = 2;
+        this._remoteVersion = 0;
+    }
+
+    setServer(server) {
+        this._server = server;
+    }
+
+    getServer() {
+        return this._server;
+    }
+
+    setEnabled(enabled) {
+        this._enabled = enabled;
+    }
+
+    isEnabled() {
+        return this._enabled;
     }
 
     async addNote(note) {
-        await this.checkVersion();
-        return await this.ankiInvoke('addNote', {note});
+        if (!this._enabled) { return null; }
+        await this._checkVersion();
+        return await this._invoke('addNote', {note});
     }
 
     async canAddNotes(notes) {
-        await this.checkVersion();
-        return await this.ankiInvoke('canAddNotes', {notes});
+        if (!this._enabled) { return []; }
+        await this._checkVersion();
+        return await this._invoke('canAddNotes', {notes});
     }
 
     async getDeckNames() {
-        await this.checkVersion();
-        return await this.ankiInvoke('deckNames');
+        if (!this._enabled) { return []; }
+        await this._checkVersion();
+        return await this._invoke('deckNames');
     }
 
     async getModelNames() {
-        await this.checkVersion();
-        return await this.ankiInvoke('modelNames');
+        if (!this._enabled) { return []; }
+        await this._checkVersion();
+        return await this._invoke('modelNames');
     }
 
     async getModelFieldNames(modelName) {
-        await this.checkVersion();
-        return await this.ankiInvoke('modelFieldNames', {modelName});
+        if (!this._enabled) { return []; }
+        await this._checkVersion();
+        return await this._invoke('modelFieldNames', {modelName});
     }
 
     async guiBrowse(query) {
-        await this.checkVersion();
-        return await this.ankiInvoke('guiBrowse', {query});
+        if (!this._enabled) { return []; }
+        await this._checkVersion();
+        return await this._invoke('guiBrowse', {query});
     }
 
     async storeMediaFile(filename, dataBase64) {
-        await this.checkVersion();
-        return await this.ankiInvoke('storeMediaFile', {filename, data: dataBase64});
+        if (!this._enabled) {
+            throw new Error('AnkiConnect not enabled');
+        }
+        await this._checkVersion();
+        return await this._invoke('storeMediaFile', {filename, data: dataBase64});
     }
 
-    async checkVersion() {
-        if (this.remoteVersion < this.localVersion) {
-            this.remoteVersion = await this.ankiInvoke('version');
-            if (this.remoteVersion < this.localVersion) {
+    async findNoteIds(notes) {
+        if (!this._enabled) { return []; }
+        await this._checkVersion();
+        const actions = notes.map((note) => ({
+            action: 'findNotes',
+            params: {
+                query: `deck:"${this._escapeQuery(note.deckName)}" ${this._fieldsToQuery(note.fields)}`
+            }
+        }));
+        return await this._invoke('multi', {actions});
+    }
+
+    // Private
+
+    async _checkVersion() {
+        if (this._remoteVersion < this._localVersion) {
+            this._remoteVersion = await this._invoke('version');
+            if (this._remoteVersion < this._localVersion) {
                 throw new Error('Extension and plugin versions incompatible');
             }
         }
     }
 
-    async findNoteIds(notes) {
-        await this.checkVersion();
-        const actions = notes.map((note) => ({
-            action: 'findNotes',
-            params: {
-                query: `deck:"${AnkiConnect.escapeQuery(note.deckName)}" ${AnkiConnect.fieldsToQuery(note.fields)}`
+    async _invoke(action, params) {
+        const result = await requestJson(this._server, 'POST', {action, params, version: this._localVersion});
+        if (isObject(result)) {
+            const error = result.error;
+            if (typeof error !== 'undefined') {
+                throw new Error(`AnkiConnect error: ${error}`);
             }
-        }));
-        return await this.ankiInvoke('multi', {actions});
+        }
+        return result;
     }
 
-    ankiInvoke(action, params) {
-        return requestJson(this.server, 'POST', {action, params, version: this.localVersion});
-    }
-
-    static escapeQuery(text) {
+    _escapeQuery(text) {
         return text.replace(/"/g, '');
     }
 
-    static fieldsToQuery(fields) {
+    _fieldsToQuery(fields) {
         const fieldNames = Object.keys(fields);
         if (fieldNames.length === 0) {
             return '';
         }
 
         const key = fieldNames[0];
-        return `${key.toLowerCase()}:"${AnkiConnect.escapeQuery(fields[key])}"`;
-    }
-}
-
-
-/*
- * AnkiNull
- */
-
-class AnkiNull {
-    async addNote() {
-        return null;
-    }
-
-    async canAddNotes() {
-        return [];
-    }
-
-    async getDeckNames() {
-        return [];
-    }
-
-    async getModelNames() {
-        return [];
-    }
-
-    async getModelFieldNames() {
-        return [];
-    }
-
-    async guiBrowse() {
-        return [];
-    }
-
-    async findNoteIds() {
-        return [];
+        return `${key.toLowerCase()}:"${this._escapeQuery(fields[key])}"`;
     }
 }
diff --git a/ext/bg/js/backend.js b/ext/bg/js/backend.js
index 04a3b932..a1b788df 100644
--- a/ext/bg/js/backend.js
+++ b/ext/bg/js/backend.js
@@ -18,7 +18,6 @@
 /* global
  * AnkiConnect
  * AnkiNoteBuilder
- * AnkiNull
  * AudioSystem
  * AudioUriBuilder
  * BackendApiForwarder
@@ -46,7 +45,7 @@ class Backend {
         this.database = new Database();
         this.dictionaryImporter = new DictionaryImporter();
         this.translator = new Translator(this.database);
-        this.anki = new AnkiNull();
+        this.anki = new AnkiConnect();
         this.mecab = new Mecab();
         this.clipboardMonitor = new ClipboardMonitor({getClipboard: this._onApiClipboardGet.bind(this)});
         this.options = null;
@@ -55,6 +54,7 @@ class Backend {
         this.audioSystem = new AudioSystem({getAudioUri: this._getAudioUri.bind(this)});
         this.audioUriBuilder = new AudioUriBuilder();
         this.ankiNoteBuilder = new AnkiNoteBuilder({
+            anki: this.anki,
             audioSystem: this.audioSystem,
             renderTemplate: this._renderTemplate.bind(this)
         });
@@ -214,7 +214,8 @@ class Backend {
             this.setExtensionBadgeText('');
         }
 
-        this.anki = options.anki.enable ? new AnkiConnect(options.anki.server) : new AnkiNull();
+        this.anki.setServer(options.anki.server);
+        this.anki.setEnabled(options.anki.enable);
 
         if (options.parsing.enableMecabParser) {
             this.mecab.startListener();
@@ -505,8 +506,7 @@ class Backend {
             await this.ankiNoteBuilder.injectScreenshot(
                 definition,
                 options.anki.terms.fields,
-                details.screenshot,
-                this.anki
+                details.screenshot
             );
         }