Sanitizing array values via express-validator schema validation

I am using the express-validator to sanitize and validate input data. On one route, the browser submits a field 'issues' as an array. I want to validate it's not empty, and sanitize the data to ensure these are all integer values.

The code below seems to successfully validate whether it's empty - but the customSanitizer never runs (I've set breakpoints on that line) and something like ["1","3a"] gets through the middleware unchanged.

app.post(
  'issuelookup',
  checkSchema({ 
   issues: {
      custom: {
         options: (value, { req, location, path }) => value.length > 0,
      },
      errorMessage: "No issues",
      customSanitizer: {
         options: (value, { req, location, path }) => {
            return value.map((v) => parseInt(v)).filter((v) => Number.isInteger(v));
         },
      },
   },
}), function (req, res, next) {
...

Can anyone explain why the customSanitzer doesn't get used, and how I can solve this without creating an entirely new middleware?