Count days between 2 dates only if the date is present in another column

Hello! I think I am missing something in this formula. It works until I add the final component of “-1”. I am trying to calculate the workdays it takes to deliver a product. I noticed that when I use the WORKDAYS formula it includes the first date in the count. I was able to correct this by adding -1.

I only want the number of days calculated if we have a payment date, otherwise I want it to be blank.

Here is my current formula:
IF({Payment Date}=“”,“”, WORKDAYS({Delivery Date},{Payment Date})-1)

Without the -1, it works perfectly. When I add the -1, I get an error message stating “one of the parameters is invalid”.

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