 This penalty is equal to 1% of the national premium index times the number of full calendar months that they were eligible for but not enrolled in Part D and did not have creditable coverage through another source.The penalty raises the premium of Part D for beneficiaries, when and if they elect coverage.In May 2018, enrollment exceeded 44 million, including both those on standalone Part D and those enrolled in a Part C plan that includes Part-D-like coverage.