Help Debugging a Transformation

I’ve stared at this for an hour and I can’t see where I’m going wrong. I am trying to rotate a point on a plane with pts(0) being my start point, pt(1) rotated 180 deg, pt(2) rotated 90 deg, and pt(3) rotated 270 deg. The center of rotation is the center of the plane.

'get an xform to translate between world and plane points
xform = Rhino.XformChangeBasis(Rhino.WorldXYPlane, plane)
'take the world coordinate and translate to planar coordinate
pts(0) = Rhino.PointTransform(Rhino.PlaneClosestPoint(plane, crvMidPt, True), xform)

I was expecting the z of pts(0) to be zero, but it is near zero, i.e. -1.2E-15.

'create a rotation xform to rotate 90 deg around the plane's center, with the z vector as the rotation axis
xform = Rhino.XformRotation(90, plane(0), plane(3))

pts(2) = Rhino.PointTransform(pts(0), xform) '90 deg from pt(0)
pts(1) = Rhino.PointTransform(pts(2), xform) '180 deg from pt(0)
pts(3) = Rhino.PointTransform(pts(1), xform) '270 deg from pt(0)

The magnitude of z’s value for pts(1) through pt(3) ranges from .08 to 1.77. Why is z changing at all? If pt(0) was (0,1,0), I would expect my remaining values to be (0,-1,0), (-1,0,0), and (1,0,0).

Edit Another 30 minutes and I got it:

xform = Rhino.XformRotation(90, Array(0, 0, 1), Array(0, 0, 0))

I needed to rotate around 0, not the world point definition of the plane’s center.