Is there an error in testing for the two-for exercise?

Programming 8th
1

I received the following error:

Exception: expected a number: at line 9 byte 166 in /mnt/exercism-iteration/test.8th <- G:eval <- a:each: task REPL

I can't see how I can avoid this.
2

It’s more likely that there is a bug in your code

3

8th forces you to be very vigilant about what you keep on the stack. At line 9 in the test.8th file, what is on the stack, what is put there by the test, and what is put there by your code?