INFO : -- Reading test list
INFO : -- Writing report to /Users/lkuskov/dev/jcov/jdk.instrument/RepGen
INFO : -- Reading data from /Users/lkuskov/dev/jcov/jdk.instrument/report/public.xml
INFO : -- Applying data processor class com.sun.tdk.jcov.processing.DefaultDataProcessorSPI
INFO : -- Starting ReportGenerator com.sun.tdk.jcov.report.html.CoverageReport
SEVERE : Illegal format argument index = 0
Exception details: java.util.IllegalFormatArgumentIndexException: Illegal format argument index = 0
at java.base/java.util.Formatter$FormatSpecifier.index(Formatter.java:2803)
at java.base/java.util.Formatter$FormatSpecifier.<init>(Formatter.java:2872)
at java.base/java.util.Formatter.parse(Formatter.java:2729)
at java.base/java.util.Formatter.format(Formatter.java:2671)
at java.base/java.io.PrintStream.format(PrintStream.java:1209)
at com.sun.tdk.jcov.report.AbstractCoverage$PercentFormatter.format(AbstractCoverage.java:202)
at com.sun.tdk.jcov.report.CoverageData.getFormattedCoverage(CoverageData.java:138)
at com.sun.tdk.jcov.report.AbstractCoverage.getCoverageString(AbstractCoverage.java:112)
at com.sun.tdk.jcov.report.html.CoverageReport.getColumnData(CoverageReport.java:909)
at com.sun.tdk.jcov.report.html.CoverageReport.printColumnCoverages(CoverageReport.java:998)
at com.sun.tdk.jcov.report.html.CoverageReport.generateSourceFile(CoverageReport.java:1098)
at com.sun.tdk.jcov.report.html.CoverageReport.generateSourceFiles(CoverageReport.java:1043)
at com.sun.tdk.jcov.report.html.CoverageReport.generate(CoverageReport.java:126)
at com.sun.tdk.jcov.report.html.CoverageReport.generateReport(CoverageReport.java:119)
at com.sun.tdk.jcov.RepGen.generateReport(RepGen.java:331)
at com.sun.tdk.jcov.RepGen.generateReport(RepGen.java:194)
at com.sun.tdk.jcov.RepGen.generateReport(RepGen.java:179)
at com.sun.tdk.jcov.RepGen.run(RepGen.java:688)
at com.sun.tdk.jcov.tools.JCovCMDTool.run(JCovCMDTool.java:159)
at com.sun.tdk.jcov.Helper.main(Helper.java:54)

The problem is caused by using argument_index == 0 that is not allowed by spec https://docs.oracle.com/en/java/javase/12/docs/api/java.base/java/util/Formatter.html#dpos
"The first argument is referenced by "1$", the second by "2$", etc. An argument may be referenced more than once."