unhandled exception: index -1 not in 0 .. 12 [IndexDefect]

[stbalbach]

Sometimes my program aborts with
    
    
    fatal.nim(49)            sysFatal
    Error: unhandled exception: index -1 not in 0 .. 12 [IndexDefect]
    
    Run

I am not handling range exceptions correctly, it is a two part question: how to 
trap exceptions of this type and, how to determine where in my code it is 
occurring.