I was able to complete the Ruby courses, so at some point did this challenge, and I typed them all in and didn't use the hints/answers. Things may've changed, or it could be a browser issue (I was using FF) ... please let me know which course, level and challenge, then I or others could try this and help diagnose the problem.
On the off chance it is a browser issue, also tell us which browser you're using.
@DavidMcGraw, I don't understand quite what you mean by "enter the code vertically" ... you mean in the Ruby course editor you can't enter code on multiple lines? Maybe it's a console-based challenge. I'd need to check it out myself. Note the code I put in is different from yours, and the difference is likely important. I had
z = Zombie.find(3), whereas you weren't giving z an initial value.