How to initialize redux slice with enum value?

I am trying to set initial state of redux slice with a enum value, but it throws Uncaught TypeError: undefined has no properties.

The code is:

export const themeTokenSlice = createSlice({
    name: "themeToken",
    initialState: darkThemeToken,
    reducers: {
        toggleThemeToken: (state) => {
            switch (state.theme) {
                case Themes.dark:
                    state = darkThemeToken;
                    break;
                case Themes.light:
                    state = lightThemeToken;
                    break;
                default:
                    return state;
            }
        }
    }
});

export const darkThemeToken: themeState = {
    theme: Themes.dark
};


export interface themeState {
    theme: Themes;
}

export enum Themes {
    dark = "dark",
    light = "light",
}

The way it works:

export const darkThemeToken: themeState = {
    theme: "dark" as Themes.dark
};

Is there a way to initialize the state with enum?