Spy on jasmine function from an Angular library is not working

Question

I have an Angular library containing functions like "export function myFunction".
The project with the error have this library as a dependency and when I want to spy on the function, the error below is display:

myFunction is not declared writable or has no setter

Now the real world example:

A simple function from the library:

export function isNotEmptyString(value: any): value is string {
  return _.isString(value) && !_.isEmpty(value);
}

The dts of this function once packaged:

export declare function isNotEmptyString(value: any): value is string;

To spy on the function, I must have an object at some point.
So I use a custom module import to achieve this.

The spy on error:

import * as MyLibfrom 'my-lib';

const isNotEmptyStringSpy = spyOn(MyLib, 'isNotEmptyString').and.returnValue(false);

And the error is:

isNotEmptyString is not declared writable or has no setter

Now, if I use a spyOnPropety:

import * as MyLibfrom 'my-lib';

const isNotEmptyStringSpy = spyOnProperty(MyLib, 'isNotEmptyString').and.returnValue(() => false);

And the new error is:

isNotEmptyString is not declared configurable

I also tried to use differents module inside the tsconfig.json compiler options.
Like commonjs, CommonJS, ES2015 and ESNext.
There is no difference.

tsconfig.json

"compilerOptions": {
  "module": "commonjs"
}

Does anyone have a suggestion?
I am stuck with this :/

Thanks !

Environment:

"karma": "4.4.1"
"jasmine-core": "3.5.0"
"@angular/core": "9.0.5"