diff --git a/src/calibre/ebooks/mobi/mobiml.py b/src/calibre/ebooks/mobi/mobiml.py
index bdf81597b1..22ee97eff2 100644
--- a/src/calibre/ebooks/mobi/mobiml.py
+++ b/src/calibre/ebooks/mobi/mobiml.py
@@ -454,8 +454,10 @@ class MobiMLizer(object):
                 text = COLLAPSE.sub(' ', elem.text)
         valign = style['vertical-align']
         not_baseline = valign in ('super', 'sub', 'text-top',
-                'text-bottom')
-        vtag = 'sup' if valign in ('super', 'text-top') else 'sub'
+                'text-bottom') or isinstance(valign, (float, int))
+        issup = valign in ('super', 'text-top') or (
+            isinstance(valign, (float, int)) and valign > 0)
+        vtag = 'sup' if issup else 'sub'
         if not_baseline and not ignore_valign and tag not in NOT_VTAGS and not isblock:
             nroot = etree.Element(XHTML('html'), nsmap=MOBI_NSMAP)
             vbstate = BlockState(etree.SubElement(nroot, XHTML('body')))