...

src/calibre/ebooks/mobi/mobiml.py

@@ -468,6 +468,7 @@ class MobiMLizer(object):
                         vtag.append(child)
                     else:
                         break
+                if vbstate.para is not None:
                     for child in vbstate.para:
                         vtag.append(child)
                 return