So it just ends at a door that you never get a code for?
I assume you're talking about the 2nd coded door in the 2: Details area, one that has a key and an empty dollhouse that only has a spring. It is possible to get the code for that door, and it happens to be on the key you carry around! Simply drop the key into the dollhouse, before jumping into it. Then hop in to make the key really large.