diff --git a/scripts/get_frequency.test.ts b/scripts/get_frequency.test.ts
new file mode 100644
index 0000000..5911864
--- /dev/null
+++ b/scripts/get_frequency.test.ts
@@ -0,0 +1,45 @@
+import assert from 'node:assert/strict';
+import fs from 'node:fs';
+import path from 'node:path';
+import test from 'node:test';
+import { spawnSync } from 'node:child_process';
+
+function createWorkspace(name: string): string {
+  const baseDir = path.join(process.cwd(), '.tmp', 'get-frequency-typecheck-test');
+  fs.mkdirSync(baseDir, { recursive: true });
+  return fs.mkdtempSync(path.join(baseDir, `${name}-`));
+}
+
+test('scripts/get_frequency.ts typechecks in isolation', () => {
+  const workspace = createWorkspace('isolated-script');
+  const tsconfigPath = path.join(workspace, 'tsconfig.json');
+
+  fs.writeFileSync(
+    tsconfigPath,
+    JSON.stringify(
+      {
+        extends: '../../../tsconfig.typecheck.json',
+        include: ['../../../scripts/get_frequency.ts'],
+        exclude: [],
+      },
+      null,
+      2,
+    ),
+    'utf8',
+  );
+
+  try {
+    const result = spawnSync('bunx', ['tsc', '--noEmit', '-p', tsconfigPath], {
+      cwd: process.cwd(),
+      encoding: 'utf8',
+    });
+
+    assert.equal(
+      result.status,
+      0,
+      `expected scripts/get_frequency.ts to typecheck\nstdout:\n${result.stdout}\nstderr:\n${result.stderr}`,
+    );
+  } finally {
+    fs.rmSync(workspace, { recursive: true, force: true });
+  }
+});
diff --git a/scripts/get_frequency.ts b/scripts/get_frequency.ts
index 5b61532..893d77e 100644
--- a/scripts/get_frequency.ts
+++ b/scripts/get_frequency.ts
@@ -484,7 +484,6 @@ interface YomitanRuntimeState {
   yomitanExt: unknown | null;
   yomitanSession: unknown | null;
   parserWindow: unknown | null;
-  yomitanSession: unknown | null;
   parserReadyPromise: Promise<void> | null;
   parserInitPromise: Promise<boolean> | null;
   available: boolean;
@@ -544,7 +543,6 @@ async function createYomitanRuntimeState(
     yomitanExt: null,
     yomitanSession: null,
     parserWindow: null,
-    yomitanSession: null,
     parserReadyPromise: null,
     parserInitPromise: null,
     available: false,