Text Frame Inset spacing and Corner Option

I can set Text Frame Inset spacing in the Text Frame Options dialogue box. But when I go back to change the settings: all the Inset Spacing for bottom, left and right are greyed out. I can't change them.

This only happens when I have applied a Corner option to the Text Frame.

When I apply any Corner option to the Text Frame, the Inset Spacing defaults to a constant for Inset, bottom, left and right: i can change only the Inset and it changes all four.

If I remove the Corner Option: I get access again to all Inset Spacing.

Thanks,

Cam

InDesign CS5