stackEl is not iterable (cannot read property undefined)

Question

I really need your help. I have a setSkillsForDropdownFilters method. From the backend comes to me a list of stacks (technologies), there is a drop-down list from which I can select technologies to sort users. The fact is that in addition to stacks, I get extraStacks and I need to sort them together in one menu. I have an application running, but I'm getting an error that looks like this:

stackEl is not iterable (cannot read property undefined)

And no sorting works for me. What am I doing wrong? Thank you very much

private setSkillsForDropdownFilters(skill: string): void {
const skillsFromArray = this.filteredUsers.map(
  (user) => (user as any)[skill]
);
if (skill === 'stack') {
  const extraStacks = this.filteredUsers.map(
    (user) => (user as any)['extraStack']
  )
    .reduce((stacksArr, stackEl) => {
      stacksArr.push(...stackEl)
      return stacksArr;
    }, [])
    .map((stack: any) => stack.title)
  skillsFromArray.push(...extraStacks);
}

if (skill === 'softSkills') {
  this.filteredSoftSkills = [...new Set(skillsFromArray.sort())];
} else {
  (this as any)[
    'filtered' + skill[0].toUpperCase() + skill.slice(1) + 's'
  ] = [...new Set(skillsFromArray.sort())];
 }
}