Flash CS3 Strings Panel : Error 1009

I’ve created a very simple Actionscript 3 FLA with the Strings Panel setting at “Automatically at Runtime” and just one language - English.

Keyframe at Frame 1: Dynamic textbox linked to a Strings Panel ID.

Keyframe at Frame 15: Another Dynamic textbox linked to another Strings Panel ID.

Compiling this setup produces the following error:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at StringsPanelNotWorking_fla::MainTimeline/StringsPanelNotWorking_fla::frame15()

If I change the Publish Settings -> Actionscript version to 2.0, the same file compiles and runs without errors.

I’ve got no code on any frames. So this means there is a problem with the dynamically generated code at runtime. Could someone tell me why such a simple thing is not working in Actionscript 3.