VB.Net: Changing control properties from outside the form

I sometimes need to set a form's control properties from elsewhere in the code, and I'm getting strange errors whenever I try.   In my situation, I've created a checkbox that I want to change, but while running from withing another form.  Here's a sample of that code:

Dim UseCheck As System.Windows.Forms.CheckBox = MyForm.MyCheckbox
With UseCheck
    .Checked = False
    .Enabled = True
End With

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With this code in place, the form won't open.
InvalidOperationException.png