Can't use own fonts

**Hi,
I tried to add custom fonts, but always with the same error. Can you advise what I am doing wrong or whether it is an internal error?**

java.lang.NullPointerException
at com.openhtmltopdf.pdfboxout.PdfBoxFontResolver.resolveFont(PdfBoxFontResolver.java:70)
at com.openhtmltopdf.layout.SharedContext.getFont(SharedContext.java:391)
at com.openhtmltopdf.pdfboxout.PdfBoxOutputDevice.processControls(PdfBoxOutputDevice.java:316)
at com.openhtmltopdf.pdfboxout.PdfBoxOutputDevice.finish(PdfBoxOutputDevice.java:1000)
at com.openhtmltopdf.pdfboxout.PdfBoxRenderer.writePDF(PdfBoxRenderer.java:607)
at com.openhtmltopdf.pdfboxout.PdfBoxRenderer.createPDF(PdfBoxRenderer.java:546)
at com.openhtmltopdf.pdfboxout.PdfBoxRenderer.createPDF(PdfBoxRenderer.java:485)
at com.openhtmltopdf.pdfboxout.PdfBoxRenderer.createPDF(PdfBoxRenderer.java:481)
at com.openhtmltopdf.pdfboxout.PdfRendererBuilder.run(PdfRendererBuilder.java:86)
at cz.codeberry.oneclick.example.SimpleUsage.exportToPdfBox(SimpleUsage.java:30)
at cz.codeberry.oneclick.example.SimpleUsage.main(SimpleUsage.java:17)

**My CSS:**
@font-face {
src: url("./OpenSans.ttf");
font-family: "Open Sans Condensed";
font-weight: 700;
}
h1 {
font-family: "Open Sans Condensed", "Arial Narrow", Arial, sans-serif;
}

**I tried I add the font directly to the builder, but also unsuccessfully:**
PdfRendererBuilder builder = new PdfRendererBuilder();
builder.withFile(file);
builder.useFont(new File("./OpenSans.ttf"), "Open Sans Condensed");
builder.toStream(os);
builder.run();