wherebetween laravel not working with variable

I am using Laravel 5.8, and I would like to get rows between two prices in my database.

I using this code to get my product:

$prd = Product::where($args)->whereBetween('price', [$request->min_price, $request->max_price])->get();

But the problem is when I use the code above and I return $prd; nothing returned but when I put a static number as $request->min_price or $request->max_price it works correctly

I must set one of them as a static number like this:

$prd = Product::where($args)->whereBetween('price', [100, $request->max_price])->get();

or:

$prd = Product::where($args)->whereBetween('price', [$request->min_price, 1000])->get();

Where is my problem or mistake?

via Chebli Mohamed