diff --git a/src/calibre/ebooks/pml/pmlml.py b/src/calibre/ebooks/pml/pmlml.py
index a6febdc53f..7c10784867 100644
--- a/src/calibre/ebooks/pml/pmlml.py
+++ b/src/calibre/ebooks/pml/pmlml.py
@@ -24,11 +24,11 @@ TAG_MAP = {
     'big'     : 'l',
     'del'     : 'o',
     'h1'      : 'x',
-    'h2'      : 'x0',
+    'h2'      : 'X0',
     'h3'      : 'x1',
-    'h4'      : 'x2',
-    'h5'      : 'x3',
-    'h6'      : 'x4',
+    'h4'      : 'X2',
+    'h5'      : 'X3',
+    'h6'      : 'X4',
     '!--'     : 'v',
 }