newbie help, changing knot type

I am trying to make a simple script to change a selected spline knot type. it goes as as follows:

a = ((getsplineselection $ )[1]) --------to get the spline integer
b = ((getknotselection $ a )[1]) --------to get the knot integer
setknottype $ a b #corner --------assigns new knot type to selection

I dragged the macro into a toolbar to make a button and it works as I need it to. The problem arises when I restart max. when I press my macroscript button again I get an error

" --unable to convert: undefined to type: integer"

so for some reason a & b are not kicking back an integer when the desired knot is selected.

In the maxscript help file the syntax for getknottype is defined as:

setKnotType (#smooth | #corner | #bezier | #bezierCorner)

Any help is greatly appreciated ! this is driving me nuts :)

also, I made a simple ui for it and the when I run the script nothing happens. I used the visual maxscript editor the final script looked something like this:

rollout knots "knot types" width:140 height:171
(
button btn1 "corner" pos:[14,32] width:107 height:24
button btn2 "smooth" pos:[15,62] width:107 height:24
button btn3 "bezier" pos:[16,96] width:107 height:24
button btn4 "bez corner" pos:[16,130] width:107 height:24
on btn1 pressed do
(
a = ((getsplineselection $)[1])
b = ((getknotselection $ a)[1])
setknottype $ a b #corner
)
on btn2 pressed do
(
a = ((getsplineselection $)[1])
b = ((getknotselection $ a)[1])
setknottype $ a b #smooth
)
on btn3 pressed do
(
a = ((getsplineselection $)[1])
b = ((getknotselection $ a)[1])
setknottype $ a b #bezier
)
on btn4 pressed do
(
a = ((getsplineselection $)[1])
b = ((getknotselection $ a)[1])
setknottype $ a b #bezierCorner
)
)

any help regarding this problem is also greatly appreciated,
Thanks,

Paul

This is your tool
--work on single selected shape (with multiple attached splines) without assigned modifiers.
--supports multiple selected knots

try(destroydialog ::knotsRoll)catch()
rollout knotsRoll "knot types"
(
fn filterSpline spl = (classof spl == Line or classof spl == SplineShape)
fn changeKnotTypes btn: =
(
if selection.count == 1 and filterSpline selection[1] and (selection[1].modifiers).count == 0 do
(
local spl = selection[1]
for s = 1 to (numSplines spl) do
(
if (knots = getKnotSelection spl s).count != 0 do
(
local type = case btn of (
(1):#corner
(2):#smooth
(3):#bezier
(4):#bezierCorner
)
for k = 1 to knots.count do setKnotType spl s knots[k] type
)
)
updateshape spl
)
)
button btn1 "Corner" pos:[5,5] width:100 height:20
button btn2 "Smooth" pos:[5,30] width:100 height:20
button btn3 "Bezier" pos:[5,55] width:100 height:20
button btn4 "BezierCorner" pos:[5,80] width:100 height:20
on btn1 pressed do (changeKnotTypes btn:1)
on btn2 pressed do (changeKnotTypes btn:2)
on btn3 pressed do (changeKnotTypes btn:3)
on btn4 pressed do (changeKnotTypes btn:4)
)
createDialog knotsRoll 110 105