It looks like you are referencing the subform, when you should be referencing the name of the control that holds the subform. To find the name of the control, go your form in design mode, click on the subform, then click on the very outermost square that surrounds the subform. That should be the control. If you are on the control, the properties window will have 2 options that say something like Parent Field Link and Child Field Link. Look at the top of the properties window, and their will be a field called Name. Whatever you have in that field should be in your code, instead of "UsageSubform".