Tk 9: image -format svg -scaletowidth not working?

I am trying to use the -format svg -scaletowidth option without success.

Given this svg file "test.svg" (copied from W3CSchools):

<svg width="100" height="100" xmlns="http://www.w3.org/2000/svg">
   <circle cx="50" cy="50" r="40" stroke="green" stroke-width="4"
fill="yellow" />
</svg>

This program works fine:

#!/usr/bin/env wish9
proc icon {svg {width 0}} {
    set opt [expr {$width ? "-format svg -scaletowidth $width" : ""}]
    image create photo -file $svg {*}$opt
}
ttk::button .button -command {destroy .} -image [icon test.svg]
grid .button

But if I want to scale the image to 24 pixels (e.g., for a menu item):

#!/usr/bin/env wish9
proc icon {svg {width 0}} {
    set opt [expr {$width ? "-format svg -scaletowidth $width" : ""}]
    image create photo -file $svg {*}$opt
}
ttk::button .button -command {destroy .} -image [icon test.svg 24]
grid .button

It fails with this error:

Error in startup script: unknown option "-scaletowidth"
    while executing
"image create photo -file $svg {*}$opt"
    (procedure "icon" line 3)
    invoked from within
"icon test.svg 24"
    invoked from within
"ttk::button .button -command {destroy .} -image [icon test.svg 24]"
    (file "./svgtest2.tcl" line 8)

Can someone tell me what I'm doing wrong?

I'm using Tcl/Tk 9.0b2 on Debian Linux.