loopOut pingpong doesn't work on a mask shape?

Hi all.

Long time AE user, new to expressions.

I'm trying to apply the loopOut("pingpong") expression to an animating mask. Doesn't seem to work. Is this an expression that does not apply to mask shapes?

(To be sure I was doing it right, after it didn't work, I used the expression on the Rotation parameter. When that worked, I copied and pasted the expression to ensure my syntax was correct.)

Thanks!

Naveen