Wondering why this gives an error

I have a for loop to build a data element for transfer to another system.

the loop builds that data out of an array. I use this type of build alot, but
this time, I get an error about an OBJECT not being part of an OBJECT or somethiing…

This works

dataOut = dataOut + "car"+zeroPad((w+1),2)+"="+consistDataArray[w][0];

This does not…

dataOut += "car"+zeroPad((w+1),2)+"="+consistDataArray[w][0];

but later in my code I do something similar and it works!

Here is the code in full…

function buildLesson1Data() {	
	linkedSCORMscript.scrollingDisplayBox += "

buildLesson1Data() STARTED";

	var dataOut:String;
	linkedSCORMscript.scrollingDisplayBox += "

consistDataArray.length=“+consistDataArray.length;
linkedSCORMscript.scrollingDisplayBox += "
consistDataArray[0][0]=”+consistDataArray[0][0];
for(var w:int=0; w<consistDataArray.length ; w++){
dataOut = dataOut + “car”+zeroPad((w+1),2)+“=”+consistDataArray[w][0];
//dataOut += “car”+zeroPad((w+1),2)+“=”+consistDataArray[w][0];
linkedSCORMscript.scrollingDisplayBox += "
“+dataOut+” has a consistDataArray[“+w+”].length=“+consistDataArray[w].length;
for(var c:int=1; c<consistDataArray[w].length ; c++){
dataOut = dataOut +”,"+consistDataArray[w]

;
					}
					dataOut = dataOut +";";
					linkedSCORMscript.scrollingDisplayBox += "
 row done ="+dataOut;
			}
			linkedSCORMscript.scrollingDisplayBox += "

dataout= "+dataOut;
			linkedSCORMscript.scrollingDisplayBox += "

buildLessonData()  ENDED";	
		return dataOut;
}


Also this function works fine, but it was built using the x +=  and works fine ? confused  : ]

    function buildLesson2Data() { //AnswersArray[dx1][1]
		//Application.ExternalCall( "callbackunity", "buildLesson2Data()  STARTED" );
		linkedSCORMscript.scrollingDisplayBox += "
buildLesson2Data()  STARTED";
		var dataOut:String;

		for(w=0; w<AnswersArray.length ; w++)
				{
					// AnswersArray[a]**  where a is the number of questions and b is 0 answerkey, 1 learners answer.
					dataOut += "Question"+zeroPad((w+1),2)+"="+AnswersArray[w][0];
					dataOut += ","+AnswersArray[w][1];
					dataOut += ";";
				}
				linkedSCORMscript.scrollingDisplayBox += "
buildLesson2Data()  ENDED";	
		return dataOut;
}

Is it only when += is followed by a “string literal” + string object; ?
Sounds like a bug. You should report it.