Invalid UTF-8 error on first argument of command line arguments

I was showing my friend some code I wrote in Rust to find how many times a word appears in a given file here’s the code

use std::fs;
use std::env;
use std::io;

const REQUIRED_ARGS: usize = 2;

fn word_in_file(word: &str, filename: &str) -> io::Result<Option<usize>> {
    let file_contents = fs::read_to_string(filename)?;

    let mut word_count: usize = 0;

    for line in file_contents.lines() {
        for text in line.split_whitespace() {
            if word == text { word_count += 1 }
        }
    } 

    match word_count {
        0 => return Ok(None),
        _ => return Ok(Some(word_count))
    }

}

fn main() {
    let cmd_args = env::args()
        //skip filepath arg because we don't need it
        .skip(1)
        .collect::<Vec<String>>();

    if cmd_args.len() < REQUIRED_ARGS { panic!("Not enough command args were passed") }

    let word = &cmd_args[0];
    let filename = &cmd_args[1];


    match word_in_file(word, filename).unwrap() {
        Some(count) => println!("Found the word {} in file {} {} times", word, filename, count),
        None => println!("Didn't find word {} in file {}", word, filename)
    }
}

My friend then told me I was being inefficient, because I didn’t need to collect the command line arguments as a vector I can just use skip and next to get the ones I want like this

let word: &String = &env::args().skip(1).next().unwrap();
let filename: &String = &env::args().skip(2).next().unwrap();

I accidentally forgot to skip the first argument and ran this code for the command arguments

let word: &String = &env::args().next().unwrap();
let filename: &String = &env::args().skip(1).next().unwrap();

And I got the error: called Result::unwrap() on an Err value: Error { kind: InvalidData, message: "stream did not contain valid UTF-8" } on line 27 where I call word_in_file(). For some reason the first command line argument when passed to fs::read_to_string() results in a invalid UTF-8 error. I tried my original code and passed the first argument of the command line arguments vector without skipping the first element and I didn’t get a invalid UTF-8 error, instead I got the expected called Result::unwrap() on an Err value: Os { code: 2, kind: NotFound, message: "The system cannot find the file specified." }

Does anyone know why this doesn’t throw a invalid UTF-8 error

let cmd_args = env::args()
        .collect::<Vec<String>>();
let first_arg: &String = &cmd_args[0];
fs::read_to_string(first_arg).unwrap(); //Expected error Err can't find file path

And why this does?

let first_arg: &String = &env::args().next().unwrap();
fs::read_to_string(first_arg).unwrap(); //Err invalid UTF-8