@Test
public void testCustomSignerFailureIfNotRegistered() throws Exception {

  Configuration config = new Configuration();
  config.set(CUSTOM_SIGNERS, "testsignerUnregistered");
  SignerManager signerManager = new SignerManager("dontcare", null, config,
      UserGroupInformation.getCurrentUser());
  // Make sure the config is respected.
  signerManager.initCustomSigners();
  // Simulate a call from the AWS SDK to create the signer.
  LambdaTestUtils.intercept(Exception.class,
      () -> SignerFactory.createSigner("testsignerUnregistered", null));
  // Expecting generic Exception.class to handle future implementation
  // changes.
  // For now, this is an NPE
}