typescript-rxjs generator call toISOString() on string format date

generator v 6.1.2

the interface clearly declare
incidentDate?: string;

but in the function it was generated as

if (incidentDate != null) { query['incidentDate'] = (incidentDate as any).toISOString().split('T')[0]; }

in swagger.json

          {
            "name": "incidentDate",
            "in": "query",
            "style": "form",
            "schema": {
              "type": "string",
              "format": "date"
            }
          }

If I passed Date object it won't compile
If I passed string I got ERROR TypeError: incidentDate.toISOString is not a function

Start with the swagger.json schema and the generated TypeScript RxJS client function that serializes incidentDate. Reproduce the mismatch using a string value and compare it with the declared interface type. Done means date-format query parameters accept the documented string type without calling an unavailable method or producing a runtime error.