diff --git a/src/calibre/ebooks/oeb/reader.py b/src/calibre/ebooks/oeb/reader.py
index 0fe46ca7e4..632cce3344 100644
--- a/src/calibre/ebooks/oeb/reader.py
+++ b/src/calibre/ebooks/oeb/reader.py
@@ -106,7 +106,7 @@ class OEBReader:
                 elem.tag = OPF(barename(elem.tag))
         nsmap.update(OPF2_NSMAP)
         attrib = dict(opf.attrib)
-        if xmlns := attrib.pop('xmlns:'):
+        if xmlns := attrib.pop('xmlns:', None):
             attrib['xmlns'] = xmlns
         nroot = etree.Element(OPF('package'),
             nsmap={None: OPF2_NS}, attrib=attrib)