Problem with pivoting when cells have missing values

I am a fairly new BI user and have been tasked with running/ upkeeping a pretty complex BI dashboard that was built by someone else.  As I was refreshing the dashboard, I received an error which seems to indicate a problem with pivoted columns when the list of values has some null cells in it.  

Here is the data prior to pivot (some things have been crossed out to preserve privacy

You can see the errors below.  How to fix this?  The dates are truly missing, so it's not an error; I can't change the missing data.