diff --git a/src/calibre/ebooks/docx/writer/container.py b/src/calibre/ebooks/docx/writer/container.py
index 82ae0bd8f7..9dd894db8b 100644
--- a/src/calibre/ebooks/docx/writer/container.py
+++ b/src/calibre/ebooks/docx/writer/container.py
@@ -223,13 +223,13 @@ class DOCX(object):
 
     @property
     def containerrels(self):
-        return textwrap.dedent(b'''\
+        return textwrap.dedent('''\
         <?xml version='1.0' encoding='utf-8'?>
         <Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships">
             <Relationship Id="rId3" Type="{APPPROPS}" Target="docProps/app.xml"/>
             <Relationship Id="rId2" Type="{DOCPROPS}" Target="docProps/core.xml"/>
             <Relationship Id="rId1" Type="{DOCUMENT}" Target="word/document.xml"/>
-        </Relationships>'''.format(**self.namespace.names))
+        </Relationships>'''.format(**self.namespace.names)).encode('utf-8')
 
     @property
     def websettings(self):