diff --git a/ass2mks.py b/ass2mks.py
index d73aa32..cfc3c2a 100755
--- a/ass2mks.py
+++ b/ass2mks.py
@@ -107,7 +107,7 @@ for font in fonts:
     command.extend(
         [
             "--attachment-mime-type",
-            mimetypes.guess_type(font["path"])[0],
+            mimetypes.guess_type(font["path"])[0] or "font/sfnt",
             "--attach-file",
             font["path"],
         ]