Trying to use .replace to get rid of a minus sign if present, but not working

HI,
I seem to be getting a 'not a function' error when using this line in the code below:
unique_opens_percent = unique_opens_percent.replace("", "-");

I'm trying to remove a dash/minus sign that may be in the unique_opens_percent variable.
Any idea where I'm going wrong? If I take out the line, the rest of the code works fine.

var unique_opens_percent = data.campaign.unique_opens / (data.previous_campaign.campaign.unique_opens/100) - 100;
	unique_opens_percent = unique_opens_percent.replace("", "-");
	unique_opens_percent = unique_opens_percent.toFixed(0);

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Thanks for any help.
tj