Why does my Javascript method return NaN when two number types are calculated?

I am learning Javascript, and I am coming across a couple of hiccups. This is my first post here, please help me understand:

1. Why does a number (ex: 2024), subtracted by my javascript object property: 'this.birthYear' (ex: 1995, clearly a number) return Not a Number (NaN)? Could this possibly be a parsing error to my browser's console?
2. And, if this is my error, what would I need to change in my code to make 2024 - 1995 = 29?

Here is my code:

const penguin = {
    firstName: 'Zachary',
    birthYear: 1995,
    job: 'Developer',
    friends: ['Michael', 'Joe', 'Evan'],
    hasDriversLicense: true,
    
    // methods
    calcAge: function () {
        this.age = 2024 - this.birthyear;
        return this.age;
    },

    getSummary: function () {
        this.hasDriversLicense ? this.getSummary = 'a' : this.getSummary = 'no';
        return this.getSummary;
    }
};

penguin.calcAge();
penguin.getSummary();

console.log(penguin.age); // why NaN ???
console.log(penguin.getSummary);

// summary

console.log(`${penguin.firstName} is a ${penguin.age} year-old ${penguin.job}, and he has ${penguin.getSummary} Driver's License.`);

Refer to screenshot for my output example.

Thank you in advance.

I have tried logging to my browser console console.log(penguin.age), which outputs NaN (But, if hard-coding a 1995 number, I get my expected result: 29).

Check the spelling of this.birthyear within your calcAge function, you want it to be this.birthYear.