diff --git a/src/calibre/ebooks/oeb/stylizer.py b/src/calibre/ebooks/oeb/stylizer.py
index 0557a1f74d..d2583b95ba 100644
--- a/src/calibre/ebooks/oeb/stylizer.py
+++ b/src/calibre/ebooks/oeb/stylizer.py
@@ -528,7 +528,7 @@ class Style(object):
                 base = parent.width
             else:
                 base = self._profile.width
-            if 'width' is self._element.attrib:
+            if 'width' in self._element.attrib:
                 width = self._element.attrib['width']
             elif 'width' in self._style:
                 width = self._style['width']
@@ -551,7 +551,7 @@ class Style(object):
                 base = parent.height
             else:
                 base = self._profile.height
-            if 'height' is self._element.attrib:
+            if 'height' in self._element.attrib:
                 height = self._element.attrib['height']
             elif 'height' in self._style:
                 height = self._style['height']