i18n format issue

geti18nLanguageString(key: string, customLanguage: Languages | string, guild?: Guild, options?: any): string {
        let language = customLanguage;
        if (guild !== undefined) {
            language = guild.language.toString();
        }
        if (options) {
            return container.i18n.format(language, key, options);
        }
        return container.i18n.format(language, key);
    }

return container.i18n.format(language, key, options);

TS2322: Type  string | TFunctionDetailedResult<string>  is not assignable to type  string 
Type  TFunctionDetailedResult<string>  is not assignable to type  string