diff --git a/src/calibre/ebooks/mobi/writer8/exth.py b/src/calibre/ebooks/mobi/writer8/exth.py
index cc8a9ca58c..d8f62a90a5 100644
--- a/src/calibre/ebooks/mobi/writer8/exth.py
+++ b/src/calibre/ebooks/mobi/writer8/exth.py
@@ -42,6 +42,7 @@ EXTH_CODES = {
     'language': 524,
     'primary_writing_mode': 525,
     'page_progression_direction': 527,
+    'override_kindle_fonts': 528,
 }
 
 COLLAPSE_RE = re.compile(r'[ \t\r\n\v]+')
@@ -221,6 +222,10 @@ def build_exth(metadata, prefer_author_sort=False, is_periodical=False,
         exth.write(ppd)
         nrecs += 1
 
+    exth.write(pack(b'>II', EXTH_CODES['override_kindle_fonts'], len(b'true') + 8))
+    exth.write(b'true')
+    nrecs += 1
+
     exth = exth.getvalue()
     trail = len(exth) % 4
     pad = b'\0' * (4 - trail)  # Always pad w/ at least 1 byte