Extend script crash when setting start/end on TrackItem

[#1]

When executing this simple code in Premiere 2020 (14.3.1 Build 45) :

var clip = app.project.activeSequence.videoTracks[0].clips[0];
clip.end = 12;

The end time of the clip changes properly but I get the error "Cannot set property end".

I tested the exact same code in Premiere 2019 (13.1.5 Build 47) and it changes the clip end without throwing any error...
Any idea why it does that ?

Many thanks

[#2]

When executing this simple code in Premiere 2020 (14.3.1 Build 45) :

var clip = app.project.activeSequence.videoTracks[0].clips[0];
clip.end = 12;

The end time of the clip changes properly but I get the error "Cannot set property end".

I tested the exact same code in Premiere 2019 (13.1.5 Build 47) and it changes the clip end without throwing any error...
Any idea why it does that ?

Many thanks