Object reference not set to an instance of an object

Sometimes when calling:

TheArtOfDev.HtmlRenderer.PdfSharp.PdfGenerator.GeneratePdf

I get this exception:

at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
at TheArtOfDev.HtmlRenderer.Core.Parse.RegexParserUtils.GetRegex(String regex) in d:\source\github\HTML-Renderer\Source\HtmlRenderer\Core\Parse\RegexParserUtils.cs:line 192
at TheArtOfDev.HtmlRenderer.Core.Parse.RegexParserUtils.Search(String regex, String source, Int32& position) in d:\source\github\HTML-Renderer\Source\HtmlRenderer\Core\Parse\RegexParserUtils.cs:line 166
at TheArtOfDev.HtmlRenderer.Core.Parse.CssParser.ParseFontProperty(String propValue, Dictionary`2 properties) in d:\source\github\HTML-Renderer\Source\HtmlRenderer\Core\Parse\CssParser.cs:line 536
at TheArtOfDev.HtmlRenderer.Core.Parse.CssParser.AddProperty(String propName, String propValue, Dictionary`2 properties) in d:\source\github\HTML-Renderer\Source\HtmlRenderer\Core\Parse\CssParser.cs:line 444
at TheArtOfDev.HtmlRenderer.Core.Parse.CssParser.ParseCssBlockProperties(String blockSource) in d:\source\github\HTML-Renderer\Source\HtmlRenderer\Core\Parse\CssParser.cs:line 414
at TheArtOfDev.HtmlRenderer.Core.Parse.CssParser.ParseCssBlockImp(String className, String blockSource) in d:\source\github\HTML-Renderer\Source\HtmlRenderer\Core\Parse\CssParser.cs:line 323
at TheArtOfDev.HtmlRenderer.Core.Parse.CssParser.FeedStyleBlock(CssData cssData, String block, String media) in d:\source\github\HTML-Renderer\Source\HtmlRenderer\Core\Parse\CssParser.cs:line 291
at TheArtOfDev.HtmlRenderer.Core.Parse.CssParser.ParseStyleBlocks(CssData cssData, String stylesheet) in d:\source\github\HTML-Renderer\Source\HtmlRenderer\Core\Parse\CssParser.cs:line 215
at TheArtOfDev.HtmlRenderer.Core.Parse.CssParser.ParseStyleSheet(CssData cssData, String stylesheet) in d:\source\github\HTML-Renderer\Source\HtmlRenderer\Core\Parse\CssParser.cs:line 100
at TheArtOfDev.HtmlRenderer.Core.Parse.CssParser.ParseStyleSheet(String stylesheet, Boolean combineWithDefault) in d:\source\github\HTML-Renderer\Source\HtmlRenderer\Core\Parse\CssParser.cs:line 81
at TheArtOfDev.HtmlRenderer.Adapters.RAdapter.get_DefaultCssData() in d:\source\github\HTML-Renderer\Source\HtmlRenderer\Adapters\RAdapter.cs:line 88
at TheArtOfDev.HtmlRenderer.Core.HtmlContainerInt.SetHtml(String htmlSource, CssData baseCssData) in d:\source\github\HTML-Renderer\Source\HtmlRenderer\Core\HtmlContainerInt.cs:line 429
at TheArtOfDev.HtmlRenderer.PdfSharp.PdfGenerator.GeneratePdf(String html, PdfGenerateConfig config, CssData cssData, EventHandler`1 stylesheetLoad, EventHandler`1 imageLoad) in d:\source\github\HTML-Renderer\Source\HtmlRenderer.PdfSharp\PdfGenerator.cs:line 119
at TheArtOfDev.HtmlRenderer.PdfSharp.PdfGenerator.GeneratePdf(String html, PageSize pageSize, Int32 margin, CssData cssData, EventHandler`1 stylesheetLoad, EventHandler`1 imageLoad) in d:\source\github\HTML-Renderer\Source\HtmlRenderer.PdfSharp\PdfGenerator.cs:line 76

It's completely random, couldn't find any pattern...

Version of Core and PdfSharp used:
`1.5.0.6`

Version of PDFSharp used:
`1.32.3057.0`