Networkdays with blank cell if criteria not met

I want to know how many days a project takes to complete. I can calculate the number of days between dates with =NETWORKDAYS(E18,L18) with E18 being the start date and L18 being the completion date. However, until a date is entered into L18 I want to the cell to be blank. The aforementioned formula returns a bogus, negative number in my destination cell. How can I cause the destination cell to be blank until a completion date is entered?