What is the test for granting immunity to a witness?

We review a district court's decision not to compel immunity for abuse of discretion, and consider whether "(1) the government has engaged in discriminatory use of immunity to gain a tactical advantage or, through its own overreaching, has forced the witness to invoke the Fifth Amendment; and (2) the witness' testimony will be material, exculpatory and not cumulative and is not obtainable from any other source." United States v. Ebbers, 458 F.3d 110, 118 (2d Cir. 2006) (internal quotation marks omitted). So "few and exceptional" are the situations in which the government is required to grant immunity to defense witnesses that "in the nearly thirty years since establishing a test for when immunity must be granted, we have yet to reverse a failure to immunize." United States v. Ferguson, 676 F.3d 260, 291 (2d Cir. 2011). Nor does this case present such a situation.