Problem with gridpicker/imageliteral

I made a customblock for a row of 12 LEDs on a I2C IOExpander. So I used imageliteral to create a clickable row of 12 LEDs.

This runs fine in Simulator, but not on the microbit. With makecode beta on Edge Win10 it generates an error when clicking Download on a paired microbit.

 //% color=#0fbc11 icon="\uf2db"
 namespace MCP23017 {

//% block="clickLED"
//% imageLiteral=1
//% imageLiteralColumns=12
//% imageLiteralRows=1
//% shim=images::createImage
export function clickLED(i: String): void {
    const im = <Image><any>i;
     im.showImage(0) // shows on LED Matrix for Test       
    // im.showImage(10)
    for (let column = 0; column < 12; column++) {
     if (im.pixel(column, 0)) {
        // setLed(column+1,State.High)
    }     else {

@peli: your testprogram Help with block field editors - large image grid picker? runs in simulator, too but not on a real micro:bit

Any Idea? Thanks!