macro-stepper raises derivation-parser error on generic-bind's (~define ...)

[jasonhemann]

To replicate on 7.4 enter the following, then click Macro Stepper

   #lang racket
   (require generic-bind)
   
   (~define x #f)

The macro stepper window should then display an error and show a [details] link.

[sorawee]

This is still an issue on 8.10.