Crash caused by adding new variable

First off all you did a really great work..

But sometimes the App on my iPad 2 crashes if i add a new variable and change it to a custom "letter".
Don't know how to post those crash reports so i actually can't link them in here. The crash appears usually
1 to 2 times per bigger equation. For example i tried it with this one:

http://upload.wikimedia.org/math/9/f/9/9f9bbb6abdfc6c8d839689359aa15bdd.png

Don't know if it depends on indizes or something. I will keep trying what could cause the Problem.