diff --git a/src/calibre/ebooks/oeb/stylizer.py b/src/calibre/ebooks/oeb/stylizer.py
index adef0f1bfb..20be750925 100644
--- a/src/calibre/ebooks/oeb/stylizer.py
+++ b/src/calibre/ebooks/oeb/stylizer.py
@@ -539,7 +539,8 @@ class Style(object):
         'Return value in pts'
         if base is None:
             base = self.width
-        font = font or self.fontSize
+        if not font and font != 0:
+            font = self.fontSize
         return unit_convert(value, base, font, self._profile.dpi)
 
     def pt_to_px(self, value):
diff --git a/src/calibre/ebooks/oeb/transforms/flatcss.py b/src/calibre/ebooks/oeb/transforms/flatcss.py
index ba90a31632..489498d90c 100644
--- a/src/calibre/ebooks/oeb/transforms/flatcss.py
+++ b/src/calibre/ebooks/oeb/transforms/flatcss.py
@@ -283,7 +283,10 @@ class CSSFlattener(object):
                 psize = fsize
             elif 'font-size' in cssdict or tag == 'body':
                 fsize = self.fmap[font_size]
-                cssdict['font-size'] = "%0.5fem" % (fsize / psize)
+                try:
+                    cssdict['font-size'] = "%0.5fem" % (fsize / psize)
+                except ZeroDivisionError:
+                    cssdict['font-size'] = '%.1fpt'%fsize
                 psize = fsize
 
         try: