diff --git a/src/calibre/ebooks/oeb/transforms/flatcss.py b/src/calibre/ebooks/oeb/transforms/flatcss.py
index 9c8442d391..c3b7f6f625 100644
--- a/src/calibre/ebooks/oeb/transforms/flatcss.py
+++ b/src/calibre/ebooks/oeb/transforms/flatcss.py
@@ -410,7 +410,10 @@ class CSSFlattener(object):
 
         if cssdict:
             for x in self.filter_css:
-                cssdict.pop(x, None)
+                popval = cssdict.pop(x, None)
+                if (self.body_font_family and popval and x == 'font-family' and
+                    popval.partition(',')[0][1:-1] == self.body_font_family.partition(',')[0][1:-1]):
+                    cssdict[x] = popval
 
         if cssdict:
             if self.lineh and self.fbase and tag != 'body':