Rust - Parallel Letter Frequency - Missing test

In this exercise, I created one HashMap by thread, and when all threads were done, I merged all the HashMap within the first HashMap .reduce as follow

    .map(|handle| handle.join().unwrap_or(HashMap::new()))
    .reduce(|mut global, local| {
        local.into_iter().for_each(|(key, value)| {
            global.entry(key).and_modify(|x| *x += value);

Notice that I used and_modify without using or_default or or_insert.

This oversight was not caught by the test suite, because it just happened that the first HashMap was already containing all the possible char.

My code shouldn’t have been validated and one way to prevent it would be to simply add a test case with each slice of &str having different char.