如何将Vec <String>扩展为HashMap值？

我有一个HashMap<String, Vec<String>>。我无法弄清楚如何通过增大来更新值Vec。我认为以下方法会起作用：

fn add_employee(mut data: HashMap<String, Vec<String>>) -> HashMap<String, Vec<String>> {
    loop {
        println!("Please enter the name of the employee you would like to manage.");

        let mut employee = String::new();

        io::stdin().read_line(&mut employee).expect(
            "Failed to read line",
        );

        let employee = employee.trim();

        let mut department = String::new();

        println!("Please enter the name of the department you would like to add the employee to.");

        io::stdin().read_line(&mut department).expect(
            "Failed to read line",
        );

        let department = department.trim();

        data.entry(department.to_string())
            .extend(vec![employee.to_string()])
            .or_insert(vec![employee.to_string()]);
    }
}

但是它却给出了错误

error: no method named `extend` found for type `std::collections::hash_map::Entry<'_, std::string::String, std::vec::Vec<std::string::String>>` in the current scope
  --> src/main.rs:27:14
   |
27 |             .extend(vec![employee.to_string()])
   |              ^^^^^^