Laravel 5: Stunned why a query is not working at the else statement with exact same parameter

I am super stunned why this works for the first, but not for the else part. I have the following function:

public function category($slug) {
    if(!isset($slug[4])) {
        $category_id = ProductCategory::where('slug', $slug[3])->first()->pluck('id');
    } else {
        $parent_id = ProductCategory::where('slug', $slug[3])->first()->pluck('id');
        $category_id = ProductCategory::where('slug', $slug[4])->where('parent_id', $parent_id)->first()->pluck('id');
    }
    return $category_id;
}

$slug is an array like this:

Array ( [0] => [1] => shop [2] => category [3] => women )

or

Array ( [0] => [1] => shop [2] => category [3] => women [4] => skirts )

When $slug[4] is not available it returns the $category_id fine, but when $slug[4] is available, the exact same query ProductCategory::where('slug', $slug[3])->first()->pluck('id') gives null, while $slug[3] is in both cases the same...

A getQueryLog() gives me this:

Array (
    [query] => select * from `product_categories` where `slug` = ? limit 1
    [bindings] => Array ( [0] => women )
)

via Chebli Mohamed