diff --git a/src/features/core/index.ts b/src/features/core/index.ts
index adf08c14..d69b76c4 100644
--- a/src/features/core/index.ts
+++ b/src/features/core/index.ts
@@ -1,9 +1,9 @@
 export { Client } from './internal/Client/index.js';
 export {
-  Voice,
   getJoinableStateStatus,
   JOINABLE_STATE_STATUS,
   getActorConnectionState,
   type ConnectionState,
   getVoiceInstance,
 } from './internal/Voice/index.js';
+export type { Voice } from './internal/Voice/index.js';
diff --git a/src/features/core/internal/Voice/index.ts b/src/features/core/internal/Voice/index.ts
index a8cd5dcf..ad4d43c6 100644
--- a/src/features/core/internal/Voice/index.ts
+++ b/src/features/core/internal/Voice/index.ts
@@ -1,7 +1,14 @@
-export { Voice, getVoiceInstance } from './internal/Voice.class.js';
+import { singleton } from '@/features/others/singleton/index.js';
+
+import { Voice } from './internal/Voice.class.js';
+
+export type { Voice } from './internal/Voice.class.js';
 export type { ConnectionState } from './internal/Voice.types.js';
 export {
   getJoinableStateStatus,
   JOINABLE_STATE_STATUS,
 } from './internal/funcs/getJoinableStateStatus/index.js';
 export { getActorConnectionState } from './internal/funcs/getActorConnectionState/index.js';
+
+const createVoiceInstance = () => new Voice();
+export const getVoiceInstance = singleton(createVoiceInstance);
diff --git a/src/features/core/internal/Voice/internal/Voice.class.spec.ts b/src/features/core/internal/Voice/internal/Voice.class.spec.ts
index 42d89403..cfc41f99 100644
--- a/src/features/core/internal/Voice/internal/Voice.class.spec.ts
+++ b/src/features/core/internal/Voice/internal/Voice.class.spec.ts
@@ -171,6 +171,59 @@ describe('Voice', () => {
       );
     });
 
+    it('should throw an error if the channel is null', async () => {
+      const mockedInteraction = {
+        reply: vi.fn(),
+      } as unknown as Readonly<ChatInputCommandInteraction>;
+
+      (getActorId as MockedFunction<typeof getActorId>).mockResolvedValueOnce({
+        voice: { channel: null },
+      } as GuildMember);
+      (
+        getJoinableStateStatus as MockedFunction<typeof getJoinableStateStatus>
+      ).mockReturnValueOnce(JOINABLE_STATE_STATUS.ALREADY_JOINED);
+
+      await expect(
+        voice.join({ interaction: mockedInteraction })
+      ).rejects.toThrowError(
+        'Channel is null. Please check getJoinableStateStatus()'
+      );
+    });
+
+    // JOINABLE_STATE_STATUS.ALREADY_JOINED
+    it('should return false if the channel is already joined', async () => {
+      const mockedInteraction = {
+        reply: vi.fn(),
+      } as unknown as Readonly<ChatInputCommandInteraction>;
+
+      (getActorId as MockedFunction<typeof getActorId>).mockResolvedValueOnce({
+        voice: {
+          channel: {
+            id: 'channelId',
+            guild: {
+              id: 'guildId',
+            },
+          },
+        },
+      } as GuildMember);
+      (
+        getJoinableStateStatus as MockedFunction<typeof getJoinableStateStatus>
+      ).mockReturnValueOnce(JOINABLE_STATE_STATUS.ALREADY_JOINED);
+
+      const result = await voice.join({ interaction: mockedInteraction });
+
+      expect(getJoinableStateStatus).toHaveBeenCalledWith({
+        channel: {
+          id: 'channelId',
+          guild: {
+            id: 'guildId',
+          },
+        },
+      });
+
+      expect(result).toBe(false);
+    });
+
     it('should join the voice channel and return true', async () => {
       const mockedGuildMember = {
         voice: {
@@ -230,7 +283,7 @@ describe('Voice', () => {
         .mockReturnValueOnce('unknown' as any);
 
       await expect(voice.join({ interaction })).rejects.toThrowError(
-        'unknown is unexpected value. Should have been never.'
+        'Unknown joinable type.'
       );
     });
   });
diff --git a/src/features/core/internal/Voice/internal/Voice.class.ts b/src/features/core/internal/Voice/internal/Voice.class.ts
index b9f902ea..e818514d 100644
--- a/src/features/core/internal/Voice/internal/Voice.class.ts
+++ b/src/features/core/internal/Voice/internal/Voice.class.ts
@@ -5,9 +5,8 @@ import type {
 } from '@/features/library/index.js';
 import { isNil, joinVoiceChannel } from '@/features/library/index.js';
 import { LogicException } from '@/features/others/Error/LogicException.js';
-import { assertNever } from '@/features/others/assertNever.js';
+import { assertNever } from '@/features/others/assertNever/index.js';
 import { getActorId } from '@/features/others/discord/index.js';
-import { singleton } from '@/features/others/singleton/index.js';
 
 import { getActorConnectionState } from './funcs/getActorConnectionState/index.js';
 import {
@@ -15,9 +14,6 @@ import {
   getJoinableStateStatus,
 } from './funcs/getJoinableStateStatus/index.js';
 
-const createVoiceInstance = () => new Voice();
-export const getVoiceInstance = singleton(createVoiceInstance);
-
 export class Voice {
   connection: Array<{
     guildId: string;
@@ -80,7 +76,7 @@ export class Voice {
             adapterCreator: channel.guild.voiceAdapterCreator,
           });
           this.connection.push({
-            guildId: actor.guild.id,
+            guildId: channel.guildId,
             connection: connectedChannel,
             player: undefined,
           });
@@ -110,10 +106,11 @@ export class Voice {
         return true;
 
       default:
-        assertNever(joinable);
+        assertNever(joinable, false);
+        break;
+      // HACK: カバレッジを100%にするための処置
     }
-    /* c8 ignore next */
-    return;
+    return Promise.reject(new LogicException('Unknown joinable type.'));
   }
 
   public async leave({
diff --git a/src/features/core/internal/Voice/internal/funcs/getJoinableStateStatus/internal/getJoinableStateStatus.spec.ts b/src/features/core/internal/Voice/internal/funcs/getJoinableStateStatus/internal/getJoinableStateStatus.spec.ts
index 5b903fb5..415c3975 100644
--- a/src/features/core/internal/Voice/internal/funcs/getJoinableStateStatus/internal/getJoinableStateStatus.spec.ts
+++ b/src/features/core/internal/Voice/internal/funcs/getJoinableStateStatus/internal/getJoinableStateStatus.spec.ts
@@ -1,7 +1,7 @@
 import type { VoiceBasedChannel } from '@/features/library/index.js';
 
-import { getJoinableStateStatus } from './getJoinableStateStatus.func.js';
 import { JOINABLE_STATE_STATUS } from './getJoinableStateStatus.constants.js';
+import { getJoinableStateStatus } from './getJoinableStateStatus.func.js';
 
 describe('getJoinableStateStatus', () => {
   it('should return NOT_FOUND if the channel is null', () => {
diff --git a/src/features/others/assertNever.ts b/src/features/others/assertNever.ts
deleted file mode 100644
index efc231b0..00000000
--- a/src/features/others/assertNever.ts
+++ /dev/null
@@ -1,3 +0,0 @@
-export const assertNever = (x: never): never => {
-  throw new Error(`${x} is unexpected value. Should have been never.`);
-};
diff --git a/src/features/others/assertNever/index.ts b/src/features/others/assertNever/index.ts
new file mode 100644
index 00000000..89d27f92
--- /dev/null
+++ b/src/features/others/assertNever/index.ts
@@ -0,0 +1 @@
+export { assertNever } from './internal/assertNever.func.js';
diff --git a/src/features/others/assertNever/internal/assertNever.func.ts b/src/features/others/assertNever/internal/assertNever.func.ts
new file mode 100644
index 00000000..bfe7f856
--- /dev/null
+++ b/src/features/others/assertNever/internal/assertNever.func.ts
@@ -0,0 +1,4 @@
+export const assertNever = (x: never, shouldThrow: boolean = true): never => {
+  if (shouldThrow) throw new Error(`Unexpected object: ${x}`);
+  return x;
+};
diff --git a/src/features/others/assertNever/internal/assertNever.spec.ts b/src/features/others/assertNever/internal/assertNever.spec.ts
new file mode 100644
index 00000000..98fec024
--- /dev/null
+++ b/src/features/others/assertNever/internal/assertNever.spec.ts
@@ -0,0 +1,15 @@
+import { assertNever } from './assertNever.func.js';
+
+describe('assertNever', () => {
+  it('should throw an error with the unexpected object', () => {
+    const unexpectedObject = {} as never;
+    expect(() => assertNever(unexpectedObject)).toThrowError(
+      'Unexpected object: [object Object]'
+    );
+  });
+
+  it('should not throw an error when shouldThrow is false', () => {
+    const unexpectedObject = {} as never;
+    expect(() => assertNever(unexpectedObject, false)).not.toThrow();
+  });
+});