jQuery returns object with length attribute undefined

I was checking if some elements exist with:

if ($(selector).length > 0){
....
}

However, sometimes the returned object (even if the element exists in the DOM and has returned) does not have the length attribute so this never evaluates to true. This error appears in chrome. Do you have any idea what the problem might be?

Edit: I use this code:

var variable;
for(let elem in selectors){ 
    if($(elem).length > 0){
      variable = true;
      break;
    }
    else
      variable = false;
  }

Given a list of selectors, variable is true if at least one of the selectors exists. This is inside a google chrome extension's content script. After this code runs in the plugin I get the same problem even in the console of google chrome.