Why did Watkins refuse to answer questions about former Communist party members?

In Watkins v. United States, Watkins had been summoned to testify before the house committee on Un-American Activities, he was asked whether he had known certain other persons to have been members of the Communist Party but Watkins refused to give information concerning individuals who had left the Communist Party, Watkins refused to answer the questions because he believed they were outside the scope of the committee's activities and that such questions violated his rights to due process under the Fifth Amendment.