Payroll tax noncompliance is exploding, the IRS inspector general for taxes warned in a report made public Wednesday.

The inspector general said that as of the end of 2015, 1.4 million employers owed approximately $45.6 billion in unpaid employment taxes, interest and penalties. Three-quarters of that was owed by employers who hadn’t paid payroll taxes in a year. That group of non-compliers has tripled in the past 17 years.