Why is null passed to my function, when called from the editor?

In following example playgroundSheet is globally defined. When run as below with the BH2, BL2:

function testDates(DateLow, DateHigh) {
  var now = playgroundSheet.getRange('BH2').getValue();;
  var nowMilli = Number(now.getTime()).toFixed(0);
  var targetDate = playgroundSheet.getRange('BL2').getValue();
  var targetDateMilli = Number(targetDate.getTime()).toFixed(0);
  var durationMilli = targetDateMilli - nowMilli;
 // return numberOfDays
 
  Logger.log(now); 
  Logger.log(nowMilli);
  Logger.log(targetDate); 
  Logger.log(targetDateMilli);
  Logger.log(durationMilli/day);
  
}

I get the results I desire:

11:00:18 AM Notice  Execution started
11:00:20 AM Info    Wed Jun 08 00:00:00 GMT-04:00 2022
11:00:20 AM Info    1654660800000
11:00:20 AM Info    Sun Jun 12 00:00:00 GMT-04:00 2022
11:00:20 AM Info    1655006400000
11:00:20 AM Info    4.0
11:00:20 AM Info    null
11:00:20 AM Info    Wed Jun 08 00:00:00 GMT-04:00 2022
11:00:20 AM Info    1654660800000
11:00:20 AM Info    Sun Jun 12 00:00:00 GMT-04:00 2022
11:00:20 AM Info    1655006400000
11:00:20 AM Info    4.0
11:00:19 AM Notice  Execution completed

But run by call to the function:

var numberOfDays = testDates('BH2','BL2');
Logger.log(numberOfDays);
function testDates(DateLow, DateHigh) {
  var now = playgroundSheet.getRange(DateLow).getValue();;
  var nowMilli = Number(now.getTime()).toFixed(0);
  var targetDate = playgroundSheet.getRange(DateHigh).getValue();
  var targetDateMilli = Number(targetDate.getTime()).toFixed(0);
  var durationMilli = targetDateMilli - nowMilli;
  return numberOfDays;

I get the following:

10:28:03 AM Notice  Execution started
10:28:03 AM Info    null
10:28:03 AM Error   
Exception: Argument cannot be null: a1Notation
testDates   @ Code.gs:13

where line 13 is

var now = playgroundSheet.getRange(DateLow).getValue();```

I have tried passing the (row, col, #row, #col) syntax with no joy and am out of ideas.

Via Active questions tagged javascript - Stack Overflow https://ift.tt/Lsq4CA9