Compiler error when putting a FxRadioButtonGroup in a VGroup container

Steps to reproduce:
1. Compile and run attached MXML file.

Actual Results:
Compiler error ahoy!

In initializer for 'mxmlContent': type mx.components.FxRadioButtonGroup is not assignable to target Array element type .

Expected Results:
No compiler error. You don't get a compiler error when you wrap the FxRadioButtonGroup in an VBox

Workaround (if any):
Put the FxRadioButtonGroup tag in a Declarations tag, or leave it in the root <Application>/<FxApplication> tag.