[racket] rackunit errors when trying to test string-math-eval

Hi,

I'm trying to create unit tests for some code that parses simple infix mathematical expressions in strings.  Despite my informal testing in the REPL showing my function 'string-math-eval' works for what I'm trying to use it for, when I try making tests using check-true or check-equal? from rackunit in a sub-module I get the error:

  +: unbound identifier;
   also, no #%app syntax transformer is bound in: +

; an example of a test which gives the error
(module+ test
  (check-true (= (string-eval-math "1+1") 2)))

I've attached my code to this message.  What is causing this to work in the main module but not in the sub-module, and in general how would I go about fixing problems like this in the future?

Regards,

Kyle Andrews
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