A minor change of code might fix it based on what i posted before...
I believe this is an error in the code. When the fence post checks to see if you can attach a lead to it it checks your inventory instead of checking if you are currently using a lead on something. However, I can't explain why you need 2 leads in your inventory. If i am right, the most obvious way to fix this bug is to make the fence check if you are using a lead, and find the line of code that requires you to have 2 leads.
Interesting.............