How to get a formula to ignore empty cells

I had a spreadsheet with a "net win/loss" column that contained the following formula:

"=E5-20"

I then used the column as source data in a chart that simply displayed net win/loss over time. As I filled data into the E column the formula would populate the "net win/loss" column and add the data point to the chart.

When I copied this sheet to make a new one for the new month, the formula copied okay, but now the net win/loss column shows (20.00) in each cell, since the E column in the new spreadsheet is empty. On the old spreadsheet the cells with the formula just stayed blank until I entered something in the corresponding E-column cell. Now, my chart shows a line at (20.00) because of all the bogus data. How do I get the new sheet to behave the same way the old one did?