How to get a count of database rows in Appian web api

I am trying to write a custom web-api in Appian that will [among other things] return a count of the number of rows in a database table. To that end, I have added this local variable in my api code.

local!countOfRows: a!queryEntity(
    entity: cons!MY_DATABASE_TABLE_DS,
    query: a!query(
      pagingInfo: a!pagingInfo(
        startIndex: 1,
        batchSize: -1
      )
    ),
    fetchTotalCount: true
  ).totalCount,

The idea is that I will then include this value as one of the output's in the json. For example:

local!dataBaseCasesWithDocs: {
    numRecs: local!countOfRows,
    recList: local!listOfRecords
}

So far the recList item works just fine - producing a nice json list of rows of data from my table [albeit 10 at a time]. But when i add the code for the countOfRows using the numRecs field, the function fails with an error 500.

Any thoughts?

thanks heaps,

David.