Assert when using nether portal in CUBES

Verification builds: Release 880513 - 1.5 BETA
Summary: Using the nether portal in CUBES results in an assert error 66% of the time

Repro Steps:
1. Download CUBES from the store (or use local files if it isnt on the store yet)
2. goto nether portal from overworld coord 585 61 -25
3. try to leave nether via portal on the other side Coord in nether are 78 72 -2.5

Observed Results: Assert pops up. The game continues afterward though.

Expected Results: Shouldn't be an assert.

Screenshot