Couldn't set above 1000m Alt

I have been running AP 4.3.0
Mission planner 1.3.79 build 1.3.8375.24878

Flying in Guided mode and try to increase altitude to 1200m AGL in action tab but mission planner automatically reset to 1000m and it cannot accept above 1000m.
How to break this function above 1000m?

Have you looked at the mission planner code?