■ Because an MDI form is always maximized, any property
  setting that attempts to change this status can also
  cause an error. Such properties include Top, Left, Width,
  and Height.
 
■ Possible solution:
 
  • Explicitly reference a non-MDI form when setting the
    value of this property.
 
■ If you need additional help, move the cursor to a keyword
  and press F1 to open the Help window.