diff --git a/libs/yggdrasil-core/src/mermaid.ts b/libs/yggdrasil-core/src/mermaid.ts
index 84438d2..b775e29 100644
--- a/libs/yggdrasil-core/src/mermaid.ts
+++ b/libs/yggdrasil-core/src/mermaid.ts
@@ -152,7 +152,7 @@ function observeBlock(pre: HTMLPreElement, render: () => Promise<void>): void {
  */
 export function initMermaid(selector: string, theme: ThemeName): Promise<void> {
   const root = document.querySelector(selector);
-  if (!root) return;
+  if (!root) return Promise.resolve();
 
   // 路径 1：未渲染的块（<code> 还在）。
   const blocks = root.querySelectorAll<HTMLPreElement>('pre > code.language-mermaid');
diff --git a/libs/yggdrasil-core/src/theme-transition.test.ts b/libs/yggdrasil-core/src/theme-transition.test.ts
index 76dabb2..53b6382 100644
--- a/libs/yggdrasil-core/src/theme-transition.test.ts
+++ b/libs/yggdrasil-core/src/theme-transition.test.ts
@@ -210,13 +210,15 @@ describe('startThemeTransition', () => {
       writable: true,
     });
 
-    // registry 回调返回一个可控的 Promise,记录它是否在 callback resolve 前完成
-    let resolveAsync: (() => void) | null = null;
+    // registry 回调返回一个可控的 Promise,记录它是否在 callback resolve 前完成。
+    // resolveAsync 用 mutable 容器包裹,绕过 TS 跨闭包的控制流收窄(否则在 ?.() 处
+    // 被判定为 null → never)。
+    const resolveAsync: { fn: (() => void) | null } = { fn: null };
     const asyncDone = { value: false };
     const off = onThemeChange((isDark) => {
       void isDark;
       return new Promise<void>((resolve) => {
-        resolveAsync = () => {
+        resolveAsync.fn = () => {
           asyncDone.value = true;
           resolve();
         };
@@ -230,7 +232,7 @@ describe('startThemeTransition', () => {
     expect(asyncDone.value).toBe(false);
 
     // 触发异步任务完成
-    resolveAsync?.();
+    resolveAsync.fn?.();
     await callbackPromise;
     expect(asyncDone.value).toBe(true);