Почему NaN? React

Мне нужно сделать сумму exercises в компоненте Total, но показывает NaN

import React from 'react'
    
const Total = (props) => {
    return(
        <>
            <p>Number of exercises {props.numOfExercises.reduce(function(previousValue, currentValue) {
               return previousValue.exercises + currentValue.exercises;
            })}</p>
        </>
    )
}
    
export default Total;



import React from 'react'
import Total from "./components/Total";

const App = () => {
    const course = 'Half Stack application development'
    const contentText = [
        { part: 'Fundamentals of React', exercises: 10 },
        { part: 'Using props to pass data', exercises: 7 },
        { part: 'State of a component', exercises: 14 }
    ];

    return (
        <div>
            <Total numOfExercises={contentText} />
        </div>
    )
}

export default App;