Help with AS 3 and Movie Clips

programming flash
1

I have a series of movie clips named Imagex_y where x is the number of the current test and y is the number of the current question.
In as 2 I have a for loop as such:

for (i=0; i<=6; i++) {
        this["MCNAME"+i+"_"+questionnumber].visible = false;
    }

When I attempt to run the code above I get an error in the compiler as such:
TypeError: Error #1010: A term is undefined and has no properties.
How can I get this code to work in as3.0?