XZ Plane: rounding an edge help

I'm trying to write the G code to round over an edge in the XZ
plane. Imagine a 1/8" thick rectangle, 1" tall, like a vice
parallel, that you want to round over the top edge on. Precision is
not critical in this instance, it is a decorative edge. Imagine
making a mark .25 down on the front face, and .25 inch across on the
top face and then cutting a rounded corner between those marks.

What happens is the edge is rounded over correctly, but then the Z
axis starts heading straight down, burying the cutter in the work
and I have to hit the emergency off button on the drive box. I'm
using Millmaster Pro for Windows which is similar to a Bridgeport
Boss 4 controller.

Perhaps the Z axis is trying to make a complete circle? Anyway,
here's the code. On the G02 move, I'm imagining a circle with a
radius of .25 inches. The center of the circle is .25 inches from
the top corner of the workpiece.

G90 (absolute mode)
G01 X0 Y0 Z0
G17 (sets XY plane)
G01 Z-.25 F5
G18 (sets XZ plane)
G02 X.25000 Z1.00000 I.25000 K.00000
G17
G01 Z.10
G01 X0 Y0

Thanks,

George