Cant figure out why my javascript wont run in IE

Ok so I have a javascript function that will show and hide tables when the "+" and "-" signs are clicked, now the function works fine in firefox but not in IE. IE reported that the last line of the function is the cause of the problem but i cant figure it out. the code is listed below, thanks for any help you can give!

function toggleMonthSection(monthID)
{
	//get status (- is shown, + is hidden)
	mstatdiv = document.getElementById('collapse_'+monthID);
	mstat = mstatdiv.innerHTML;

	if(mstat == '-')
	{
		var mstyle = 'none';
		var mcol = '+';
	}
	else
	{
		var mstyle = 'table';
		var mcol = '-';
	}
	mdiv = document.getElementById('table_'+monthID);
	
	mstatdiv.innerHTML = mcol;
	mdiv.style.display = mstyle;
}

Code (markup):

 

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