diff --git a/src/calibre/ebooks/oeb/transforms/flatcss.py b/src/calibre/ebooks/oeb/transforms/flatcss.py
index 489498d90c..62949b5fef 100644
--- a/src/calibre/ebooks/oeb/transforms/flatcss.py
+++ b/src/calibre/ebooks/oeb/transforms/flatcss.py
@@ -54,6 +54,8 @@ class KeyMapper(object):
         endp = 0 if size < base else 36
         diff = (abs(base - size) * 3) + ((36 - size) / 100)
         logb = abs(base - endp)
+        if logb == 0:
+            logb = 1e-6
         result = sign * math.log(diff, logb)
         return result