Unexpected Type Error on Properties.getValue in the background process

I get an Unexpected Type Error on Properties.getValue in the simulator (SDK 8.3.0, fr55)

<property id="kV" type="number">0</property>

The key passed to getValue is "kV" and I can see in the Memory View that the value is Number: 0.

Also we're talking about the uncatchable Error, not one of the 2 exceptions that can be caught (I have a try / catch and it doesn't get there)

Any idea why I get the error? It is called in a background process, but as far as I know I should be able to read the value there. Also the same getValue("kV") works in the foreground process.

(:background)
function getConfig(key as PropertyKeyType) as PropertyValueType or Null {
  var val;
  if (LOG) {
    log("getConfig: " + key);
  }
  try {
    val = Properties.getValue(key); // error on this line
  } catch (e) {
    if (LOG) {
      log(key + ":" + e.getErrorMessage());
    }
    val = null;
  }
  return val;
}
Error: Unexpected Type Error
Details: Failed invoking <symbol>