Mono.defaultIfEmpty is called even when the object is not empty

Here is the code snippet

public Mono<ResponseEntity<Response>> validateUser(@Valid @RequestBody ValidateUserRequest request, BindingResult errors){

Mono<Response> response = null;
try
{
    response = myservice.validateUser(request);
    return response.map(myresponse -> {
       if(null!=myresponse.getError(){
           return ResponseEntity.status(HttpStatus.OK).body(myresponse);
           }).defaultIfEmpty(ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR)
               .body(transformer.tranformDefaultResponse());
          }
} catch(Exception e) {
     return mono.just(ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR)
            .body(transformer.tranformExceptionResponse());
}

tranformDefaultResponse is called immediately after myservice.validateUser(request);
always even though the response is not empty. How can we prevent that from happening? I want defaultIfEmpty to be called only when the response is empty.