a!queryProcessAnalytics Task Report cannot filter "The report column specified for the additional filter is not available. (APNX-1-4156-001)"

Question

So the shape of my data returned by my Task Report by Process Model has column "c2". Here is 1 row: 
 [c3:6/20/2017 4:29 PM GMT+00:00,c4:cody.smith@itgfirm.com,c0:Pegasys Errors,c1:1,c2:Update Commit And Reservation To Pegasys (EQOFC4571-17-0001),dp0:,dp1:,dp2:,dp3:,dp4:]; 
 But when I add filter: a!queryFilter(field: "c2", operator: "=", value: "a") to my a!query, I get this error: 
 [startIndex=0, batchSize=0, sort=, totalCount=0, data=, identifiers=, name=Generic Tasks by Process Model, description=, columnConfigs=, errorMessage=The report column specified for the additional filter is not available. (APNX-1-4156-001)] 
 I do NOT have this issue with my Processes by Process Model report, which does contain filters. I ONLY get the issue with Tasks by PM. 
 
 This is very unfortunate because it appears I have to apply filters on the result, instead of filtering directly. This may have performance and scaling issues.

codys · Answer

The issue was a misunderstanding that passing {} to the filter key of a!query constructs a bad filter. null is the expected result. Our solution is to build a simpler query constructor utility since the OOTB a!query is overcomplicated.