A federal statute entitles witnesses in federal cases to a modest fee ($40 per day) for the time and trouble of testifying, along with mileage and subsistence allowances. 28 U.S.C. Secs. 1821(b)-(d). (And out of pocket travel expenses, Secs. (c)(1), (c)(3), but they are not at issue here.) But if the witness is a prisoner, he is not entitled to the fee or the allowances. 28 U.S.C. Sec. 1821(f). In this case a prisoner was transferred from a federal prison, pursuant to a federal government subpoena, to a county jail, where he remained for 27 days as a potential witness, though he wasn't actually called. He was refused a witness fee, and challenges the constitutionality of section 1821(f). His challenge presents an issue of first impression but little difficulty. Although the federal government has been held to have the same duty under the due process clause of the Fifth Amendment to avoid arbitrary classifications as the states have under the equal protection clause of the Fourteenth Amendment, Schweiker v. Wilson, 450 U.S. 221, 226 n. 6, 101 S. Ct. 1074, 1079 n. 6, 67 L. Ed. 2d 186 (1981), all that this requires, so far as treating prisoners differently from nonprisoners is concerned, is that the government avoid irrational classifications. Pryor v. Brennan, 914 F.2d 921, 923 (7th Cir.1990). It is hardly irrational to deny fees and allowances to prisoner witnesses. Their time is the government's. If the government decides that some of it shall be spent in the witness box, they are no more entitled to compensation than they are entitled to be compensated for time spent cooling their heels in a prison cell.

AFFIRMED.