Are there any rules for which words should be switched from 'ie' to 'ei'?

I'm passing all the tests except some of the random tests which are jumbles of letters/non real words. Some of them want the switch, others don't..

Thanks!

I pass all the tests, but keep getting the following --

"find_it': undefined method []' for nil:NilClass (NoMethodError)
from block in ' from block in describe'
from measure' from describe'
from `"

I ran the program in my IDE with the example tests found with the problem and have not gotten any errors. Any help would be appeciated!
'