Displaying Zero in Indicator

I have an indicator on my dashboard, showing the daily change in cumulative subscription sales. When no sales were made on the prior day, the indicator just displays a dollar sign. I'd prefer it to display a zero.

I've tried revising the formula to:

IF(IsNull(
(sum([Total Ticket Paid/Reserved Value]), [Days in Date]))
, 0
, (sum([Total Ticket Paid/Reserved Value]), [Days in Date])
)

Analytics accepts this formula, but this results in a yellow error icon being displayed when the formula dialog is closed.

What am I doing wrong? Is there another way to accomplish this? I'm pretty sure my parentheses are correct.

Thanks!

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