Having a bit of trouble here and wondering if anyone can help..

I'm trying to use dict.strip to remove unused keys in a pattern editor patch- user can set a range of keys to work in with a number box but will leave unused ones if they lower the range.

I'm using the attrui object to set the keys to strip, but what it takes in seems to be quite unpredictable...missing out first item or duplicating items. I can't tell if this is down to something I'm doing wrong or if it's a bug.

Here's the problem area: