[plt-scheme] Using DrScheme Debugger

Hi everyone,

I'm new to Scheme and just start using DrScheme as the IDE. I still
have trouble using the debug feature of DrScheme.

When run normally, I click on the Run button and then enter some
expression such as (fact 2) in the interaction window and it will
return the result to me. However, when I try to debug by click on the
debug button, and then enter the same expression in the interaction
window, I got the following error message:

send: target is not an object: #f for method: get-tab

I'm sure that I must do something fundamentally wrong but I could not
figure it out. I hope you could tell me what it is or what is the
right way to use the debug function.

Thank you in advance.
ejel