Custom Export and Maths



I'm trying to create a column that shows me the remaining Credit Balance on our user's credit cards (non-expensify cards). I have a column pulling in the Report Total and another column bringing in Custom Field 1 (This is set as the users Credit Limit). Therefore, I want to use the below formula however it is not working. I've tried it with a few different ways but not getting anything.

{math: {report:submit:from:customfield1} - {report:total}}

I get the feeling it may be due to Customfield1 as i'm not sure if this is a numerical field.

Thanks in advance.