Strict typing | Type Mismatch - But Why?

function myFunction():Void {
 var myMC:MovieClip = this.createEmptyMovieClip("mc", this.getNextHighestDepth());
 var myTxt:TextField = myMC.createTextField("txt", 0, 20, 20, 0, 0);

The above function returns the error below:

Type mismatch in assignment statement: found Void where TextField is required.

But I don’t understand why.

As far as a work-around, I can just reference the ‘mc’ movieclip by its actual instance name, that wont be a problem as of yet. but if there was a situation where i wanted to reference the movieclip by this ‘myMC’ variable, like i’m attempting to above, how would it be done?

thanks for any help in advance.