src/Resolver.php
public function castKey($key, string $default = ""): string
    {
        return is_int($key) || empty($key) ? $default : (string)$key;
    }
public function castKey($key, string $default = ""): string
    {
        return is_int($key) || empty($key) ? $default : (string)$key;
    }
The key should be a native string, so it will cast what it can with a fallback.
The goal of this is to normalize associative keys and allowing the developer to
decide if they want to included non-associative keys.

@param mixed $key
@param string $default
@return string