Month-Over-Month Change When Missing Value for Month

I have a pivot table that calculates the price per pound (PPP) of resin by month.

PPP = DIVIDE ( Sum(PO Dollar Amount),Sum(PO Quantity))

I am looking to calculate the percent change from month to month.

LastMonthPPP = CALCULATE([PPP],DATEADD('Calendar[Date]',-1,MONTH)
Month Over Month % Change = (PPP - LastMonthPP) / LastMonthPPP

However, some months we do not buy any product.

Price Per Pound Missing Months.png

The result is "NUM" errors and false "-100%" results because of the lack of a values for every month.

Price Per Pound Errors.png

I believe I need to use a variable to store and recall the last valid PPP but cannot fully grasp this concept yet.