not saying it will work, but you can try,

my.tilt = clamp(0, my.tilt, 0);

I suggest this because from my thinking,
my.x = my. pan (x is the first element in the position vector, and pan is the first element in the Angle vector)
my.y = my. tilt (y is the second element in the position vector, and tilt is the second element in the angle vector)
my.z = my. roll

just a thought...