Why is typescript declaring my parameter as undefined?

In the following function I am trying to call the ProductServices's getSelectProduct method.

private getSelectedProduct = () => {
const productId = this.activatedRoute.snapshot.paramMap.get('productId');
console.log('selectedProduct', productId)
this.productService.getSelectedProduct(productId).subscribe(product => {
this.selectedProduct = product;
})
}


Typescript is giving me a red squiggly under the productId in this statement: getSelectedProduct(productId). The error message is:

Argument of type 'string | null' is not assignable to parameter of type 'string'. Type 'null' is not assignable to type 'string'.ts(2345) const productId: string | null
​

This is the declaration of selectedProduct: public selectedProduct?roductModel;

This is the getSelectedProduct method:

public getSelectedProduct = (productId:string) => {
const product = this.productSignal().products.find(product => product._id === productId)
return of(product)
}

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