1. create a new file > new part > open sketcher.
2. draw a vertical line (vertical constraint)
3. add a vertical length constraint of 4200 mm
4. draw a new horizontal line from the bottom point to the right (horizontal constraint)
5. add a horizontal length constraint of 10000 mm (10m)

12:28:18  Traceback (most recent call last):
  File "<string>", line 1, in <module>
<class 'ValueError'>: Datum 10,00 m for the constraint with index 6 is invalid
12:28:18  App.getDocument('Naamloos').getObject('Sketch').setDatum(6,App.Units.Quantity('10000.000000 mm'))
12:28:30  Traceback (most recent call last):
  File "<string>", line 1, in <module>
<class 'ValueError'>: Datum 10,00 m for the constraint with index 6 is invalid
12:28:30  App.getDocument('Naamloos').getObject('Sketch').setDatum(6,App.Units.Quantity('10000.000000 mm'))