checkNumberOfArguments() expects 'source' to be set

getControllerArguments() appears to allow for the 'source' of a resource argument to not be set:

    // Map source data to arguments.
    $arguments = array();
    if (isset($controller['args'])) {
      foreach ($controller['args'] as $i => $info) {
        // Fill in argument from source
        if (isset($info['source'])) {

However, checkNumberOfArguments() does not:

    if (isset($resource_operation['args'])) {
      foreach ($resource_operation['args'] as $argument) {
        if (is_array($argument['source']) && isset($argument['source']['path'])) {