Divide by zero only in a range variable

I'm attaching a Prime 1.0 worksheet that demonstrates a problem I'm having. I'm defining a function with a program which tests for a zero input. This works fine when you invoke the function with a constant or a simple variable of zero, but not when the argument is a range variable including a zero value. Am I doing something wrong, or is there a workaround to this?

Thanks,

Gerrit