JSD_EvaluateScriptInStackFrame and "var" variables

Hello!

Again me asking dumb questions. I'm stuck in the situation when I try
to do JSD_EvaluateScriptInStackFrame() while executing some hook
handler (interrupt hook and execution hook). This is the way I try to
implement "Evaluate/Modify" and "Watch" features of the debugger. The
problem arises if I try to evaluate something which contains "var"-ed
variables from the script being debugged. SpiderMonkey seems to report
that their values are undefined. I believe that the problem is somehow
related with "variables object" and "this object" as stated in the
comment to JS_EvaluateScript() in jsapi.h. However, I still can't
understand what is going on here and how to behave in such situation.
Could someone explain me?