$typography_defaults : Array to string conversion problem

I already have read another question of another person about this problem.

Notice: Array to string conversion in /XXXXXXX~~~

I guess this message displays because I turned on error_reporting, but I want to resolve this for the future customer of my developing theme.

I guess it’s caused by $typography_defaults variable, as it is displayed only at the typography-related options, especially which has /’std’ => $typography_defaults,/. So I think ‘std‘ is a string and $typography_defaults is an array, and you could fix this problem by converting $typography_defaults array to string or something explicitly. I’m a newbie for PHP language, so please fix this errors.

https://ww.wp.xz.cn/plugins/options-framework/