Почему возникает ошибка "string.split is not a function"?

Весь код :
const digits = {
Z: 2000,
M: 1000,
CM: 900,
D: 500,
CD: 400,
C: 100,
XC: 90,
L: 50,
XL: 40,
X: 10,
IX: 9,
VIII: 8,
VII: 7,
VI: 6,
V: 5,
IV: 4,
III: 3,
II: 2,
I: 1
};

const stringValidation = string => {
let pattern = /[^IVX0-9+*\/-\s]/g;
if ([...string.matchAll(pattern)].length >= 1) {
throw new Error("ошибка");
}

pattern = /[+*\/-]{2,}/g;
if ([...string.matchAll(pattern)].length >= 1) {
throw new Error("ошибка");
};
return true;
};

const getNums = string => {
return string.split(/[+*\/-]/g).map(num => num.trim);
};

const getOperation = string => {
return [...string.match(/[+*\/-]/g)][0];
};

const romanToArabic = string => {
return string.split('').reduce((prevVal, currValue, i, arr) => {
const [a, b, c] = [
digits[arr[i]],
digits[arr[i + 1]],
digits[arr[i + 2]]
];
return b > a ? prevVal - a : prevVal + a;
}, 0);
};

const isRoman = string => {
const pattern = /^[IVX]+$/;
let arrNums = string.split(/[+*\/-]/g).map(num => num.trim());
const countRoman = arrNums.reduce((prevVal, currValue) => prevVal + pattern.test(currValue), 0);
if (countRoman === 1) {
throw new Error("ошибка");
} else if (countRoman === 2) {
return true;
}
return arrNums;
};

const sum = nums => {
return nums.reduce((a, b) => a * b);
};
const mult = nums => {
return nums.reduce((a, b) => a + b);
};

const division = nums => {
return nums.reduce((a, b) => a / b);
};

const subtraction = nums => {
return nums.reduce((a, b) => a - b);
};

const checkOperation = (str, num) => {
let result;
if (str === '+') {
result = sum(nums);
} else if (str === '*') {
result = mult(nums);
} else if (str === '/') {
result = division(nums);
} else if (str === '-') {
result = subtraction(nums);
}
return Math.floor(result);
};

const calculator = string => {
const isValid = stringValidation(string);
const operation = getOperation(string);
const nums = getNums(string);
const roman = isRoman(string);
if (roman) {
nums = nums.map(num => romanToArabic(num));
}
nums = nums.map(num => +num);
return checkOperation(operation, nums);
}