Trying to generate image using html format, gives java.lang.UnsupportedOperationException!

Question

The command executed is:

:=> java -jar plantuml.jar -thtml mySeq1.txt

and the result is the following. I get a html-file with zero-size:

java.lang.UnsupportedOperationException: HTML
    at net.sourceforge.plantuml.ugraphic.ImageBuilder.createUGraphic(ImageBuilder.java:263)
    at net.sourceforge.plantuml.ugraphic.ImageBuilder.writeImageTOBEMOVED(ImageBuilder.java:146)
    at net.sourceforge.plantuml.ugraphic.ImageBuilder.writeImageTOBEMOVED(ImageBuilder.java:127)
    at net.sourceforge.plantuml.sequencediagram.graphic.SequenceDiagramFileMakerPuma2.createOne(SequenceDiagramFileMakerPuma2.java:237)
    at net.sourceforge.plantuml.sequencediagram.SequenceDiagram.exportDiagramInternal(SequenceDiagram.java:220)
    at net.sourceforge.plantuml.UmlDiagram.exportDiagram(UmlDiagram.java:229)
    at net.sourceforge.plantuml.PSystemUtils.exportDiagramsSequence(PSystemUtils.java:176)
    at net.sourceforge.plantuml.PSystemUtils.exportDiagrams(PSystemUtils.java:66)
    at net.sourceforge.plantuml.SourceFileReader.getGeneratedImages(SourceFileReader.java:210)
    at net.sourceforge.plantuml.Run.manageFileInternal(Run.java:394)
    at net.sourceforge.plantuml.Run.processArgs(Run.java:316)
    at net.sourceforge.plantuml.Run.manageAllFiles(Run.java:291)
    at net.sourceforge.plantuml.Run.main(Run.java:135)
Exception in thread "main" java.lang.UnsupportedOperationException: HTML
    at net.sourceforge.plantuml.ugraphic.ImageBuilder.createUGraphic(ImageBuilder.java:263)
    at net.sourceforge.plantuml.ugraphic.ImageBuilder.writeImageTOBEMOVED(ImageBuilder.java:146)
    at net.sourceforge.plantuml.ugraphic.ImageBuilder.writeImageTOBEMOVED(ImageBuilder.java:127)
    at net.sourceforge.plantuml.UmlDiagram.exportDiagramError2(UmlDiagram.java:278)
    at net.sourceforge.plantuml.UmlDiagram.exportDiagramError(UmlDiagram.java:244)
    at net.sourceforge.plantuml.UmlDiagram.exportDiagram(UmlDiagram.java:237)
    at net.sourceforge.plantuml.PSystemUtils.exportDiagramsSequence(PSystemUtils.java:176)
    at net.sourceforge.plantuml.PSystemUtils.exportDiagrams(PSystemUtils.java:66)
    at net.sourceforge.plantuml.SourceFileReader.getGeneratedImages(SourceFileReader.java:210)
    at net.sourceforge.plantuml.Run.manageFileInternal(Run.java:394)
    at net.sourceforge.plantuml.Run.processArgs(Run.java:316)
    at net.sourceforge.plantuml.Run.manageAllFiles(Run.java:291)
    at net.sourceforge.plantuml.Run.main(Run.java:135)