diff --git a/src/config/__tests__/toml-loader.test.ts b/src/config/__tests__/toml-loader.test.ts
index a201f02..84863e6 100644
--- a/src/config/__tests__/toml-loader.test.ts
+++ b/src/config/__tests__/toml-loader.test.ts
@@ -1091,6 +1091,20 @@ timezone = "Asia/Seoul"
         expect(() => loadTomlConfig()).toThrow('invalid timezone');
       });
 
+      it('should throw error for non-string timezone (TOML array)', () => {
+        // ["local"] coerces to the string "local" via RegExp.test(), so the
+        // typeof guard is required to reject it before it reaches the driver.
+        const tomlContent = `
+[[sources]]
+id = "test_db"
+dsn = "mysql://user:pass@localhost:3306/testdb"
+timezone = ["local"]
+`;
+        fs.writeFileSync(path.join(tempDir, 'dbhub.toml'), tomlContent);
+
+        expect(() => loadTomlConfig()).toThrow('invalid timezone');
+      });
+
       it('should work without timezone (optional field)', () => {
         const tomlContent = `
 [[sources]]
diff --git a/src/config/toml-loader.ts b/src/config/toml-loader.ts
index 04f5d42..07ccb8b 100644
--- a/src/config/toml-loader.ts
+++ b/src/config/toml-loader.ts
@@ -457,7 +457,10 @@ function validateSourceConfig(source: SourceConfig, configPath: string): void {
         `Configuration file ${configPath}: source '${source.id}' has 'timezone' but it is only supported for MySQL and MariaDB sources.`
       );
     }
-    // Accepted by mysql2/mariadb drivers: "local", "Z", or "±HH:MM" (e.g., "+09:00")
+    // Accepted by mysql2/mariadb drivers: "local", "Z", or "±HH:MM" (e.g., "+09:00").
+    // The typeof guard is required: TOML can yield non-strings (e.g. arrays), and
+    // RegExp.test() would coerce a single-element array like ["local"] to a passing
+    // string before it reaches the driver as a non-string value.
     if (
       typeof source.timezone !== "string" ||
       !/^(?:local|Z|[+-]\d\d:\d\d)$/.test(source.timezone)
diff --git a/src/connectors/__tests__/mysql.integration.test.ts b/src/connectors/__tests__/mysql.integration.test.ts
index 3957546..ff8cbee 100644
--- a/src/connectors/__tests__/mysql.integration.test.ts
+++ b/src/connectors/__tests__/mysql.integration.test.ts
@@ -407,7 +407,8 @@ describe('MySQL Connector Integration Tests', () => {
       const connector = new MySQLConnector();
       try {
         // With timezone "+09:00", the driver reads the naive DATETIME as KST and
-        // produces the correct UTC instant: 02:31:23 KST == 17:31:23 UTC.
+        // produces the correct UTC instant. KST is UTC+9, so 02:31:23 on Sep 29
+        // is 17:31:23 UTC on the previous day (Sep 28).
         await connector.connect(mysqlTest.connectionString, undefined, {
           timezone: '+09:00',
         });
@@ -419,7 +420,7 @@ describe('MySQL Connector Integration Tests', () => {
 
         expect(result.rows).toHaveLength(1);
         const iso = new Date(result.rows[0].dt as string | Date).toISOString();
-        expect(iso).toBe('2025-09-29T17:31:23.000Z');
+        expect(iso).toBe('2025-09-28T17:31:23.000Z');
       } finally {
         await connector.disconnect();
       }