How to assign string | undefined to string in TypeScript?

I want to assign a variable, which is string | undefined, to a string variable, as you see here:

private selectedSerialForReplace(): string | undefined {
    return this.selectedSerials.pop();
  }

luminaireReplaceLuminaire(params: {  "serial": string; "newserial": string; }, options?: any): FetchArgs {
............
}

luminaireReplaceLuminaire({serial: this.selectedSerialForReplace(), newserial: response.output});

I get this error:

Argument of type '{ serial: string | undefined; newserial: any; }' is not assignable to parameter of type '{ "serial": string; "newserial": string; }'

I cannot change selectedSerialForReplace() function to return anything else. Could you please help me?