Unexpected missing generic font family

Using the "Check book" function throws up the following error: "Unexpected missing generic font family (font-family-no-missing-generic-family-keyword)."

For the following CSS declaration: span.font {font-family: "KaiTi";}

But the font is showing as available in the "Fonts" list; and "Manage fonts" also shows that the font is available and embedded.

This is happening regularly with embedded fonts in downloaded ebooks. What is causing the error and how to resolve it?