When I use a separate function like isNum, TypeScript throws wrong red highlights in vscode. (compiler error)

for instance,

// in some file 

const print = (param: number) => {
  console.log(param);
};

const isNum = (num: unknown): boolean => {
  return typeof num === 'number';
};

if (isNum(a)) print(a);

// in some other file
export const a: number | null = 1;

a in print(a) is red highlighted and it says “Argument of type ‘number | null’ is not assignable to parameter of type ‘number’.”
I believe it shouldn’t happen because isNum function ensures it returns true when the given value is number.

The weird thing is when it’s inline, it works

const print = (param: number) => {
  console.log(param);
};

if (typeof a === 'number') print(a);

could you tell me why it is? am i missing something?
and could you recommend good way to resolve this?
fyi, I don’t want to use type assertion. because I think it harms readability.
I don’t want to do inline checking neither.

my tsconfig file

{
  "compilerOptions": {
    "module": "commonjs",
    "declaration": true,
    "removeComments": true,
    "emitDecoratorMetadata": true,
    "experimentalDecorators": true,
    "allowSyntheticDefaultImports": true,
    "target": "ES2021",
    "sourceMap": true,
    "outDir": "./dist",
    "baseUrl": "./",
    "incremental": true,
    "skipLibCheck": true,
    "strictNullChecks": true,
    "noImplicitAny": false,
    "strictBindCallApply": true,
    "forceConsistentCasingInFileNames": true,
    "noFallthroughCasesInSwitch": false,
    "esModuleInterop": true,
    "noEmitOnError": true
  },
}

Thank you!