Calculation of dates, return blank when one of the dates is not populated

Hello,

I'm trying to calculate the time between dates.

My current formula is =SUM(B1-A1+1)
using the 1 to show 15 Dec 14 - 15 Dec 12 is equal to 1 day

My problem is when the date in either A1 or B1 has NOT been populated yet (note these are populated from another formula vlookup) the above formula is returning a value of 1. Need the formula to return a blank in this instance.

Thanking you very much