diff --git a/src/docx/__init__.py b/src/docx/__init__.py
index bbd7267c4..ba58680cf 100644
--- a/src/docx/__init__.py
+++ b/src/docx/__init__.py
@@ -13,7 +13,7 @@
 if TYPE_CHECKING:
     from docx.opc.part import Part
 
-__version__ = "1.0.0"
+__version__ = "1.0.1-dev"
 
 
 __all__ = ["Document"]
diff --git a/src/docx/opc/customprops.py b/src/docx/opc/customprops.py
index 83eea5844..3b67ed82b 100644
--- a/src/docx/opc/customprops.py
+++ b/src/docx/opc/customprops.py
@@ -10,8 +10,7 @@
 
 import numbers
 from lxml import etree
-
-NS_VT = "http://schemas.openxmlformats.org/officeDocument/2006/docPropsVTypes"
+from docx.oxml.ns import nspfxmap, qn
 
 
 class CustomProperties(object):
@@ -26,12 +25,12 @@ def __getitem__(self, item):
         prop = self.lookup(item)
         if prop is not None:
             elm = prop[0]
-            if elm.tag == f"{{{NS_VT}}}i4":
+            if elm.tag == qn("vt:i4"):
                 try:
                     return int(elm.text)
                 except ValueError:
                     return elm.text
-            elif elm.tag == f"{{{NS_VT}}}bool":
+            elif elm.tag == qn("vt:bool"):
                 return True if elm.text == '1' else False
             return elm.text
 
@@ -45,8 +44,8 @@ def __setitem__(self, key, value):
             elif isinstance(value, numbers.Number):
                 elm_type = 'i4'
                 value = str(int(value))
-            prop = etree.SubElement(self._element, "property")
-            elm = etree.SubElement(prop, f"{{{NS_VT}}}{elm_type}", nsmap={'vt':NS_VT})
+            prop = etree.SubElement(self._element, qn("op:property"), nsmap=nspfxmap("op"))
+            elm = etree.SubElement(prop, qn(f"vt:{elm_type}"), nsmap=nspfxmap("vt"))
             elm.text = value
             prop.set("name", key)
             # magic number "FMTID_UserDefinedProperties"
@@ -55,9 +54,9 @@ def __setitem__(self, key, value):
             prop.set("pid", str(len(self._element) + 1))
         else:
             elm = prop[0]
-            if elm.tag == f"{{{NS_VT}}}i4":
+            if elm.tag == qn("vt:i4"):
                 elm.text = str(int(value))
-            elif elm.tag == f"{{{NS_VT}}}bool":
+            elif elm.tag == qn("vt:bool"):
                 elm.text = str(1 if value else 0)
             else:
                 elm.text = str(value)
diff --git a/src/docx/oxml/__init__.py b/src/docx/oxml/__init__.py
index e385badd8..c1b3146e0 100644
--- a/src/docx/oxml/__init__.py
+++ b/src/docx/oxml/__init__.py
@@ -90,7 +90,7 @@
 
 from .customprops import CT_CustomProperties  # noqa
 
-register_element_cls('cup:Properties', CT_CustomProperties)
+register_element_cls('op:Properties', CT_CustomProperties)
 
 from .document import CT_Body, CT_Document  # noqa
 
diff --git a/src/docx/oxml/customprops.py b/src/docx/oxml/customprops.py
index 506645289..6f1605a59 100644
--- a/src/docx/oxml/customprops.py
+++ b/src/docx/oxml/customprops.py
@@ -16,21 +16,19 @@
 from docx.oxml import parse_xml
 
 
-
 class CT_CustomProperties(BaseOxmlElement):
     """
     ``<Properties>`` element, the root element of the Custom Properties
     part stored as ``/docProps/custom.xml``. String elements are
     limited in length to 255 unicode characters.
     """
-
-    _customProperties_tmpl = "<cup:Properties %s/>\n" % nsdecls("cup", "vt")
+    _customProperties_tmpl = "<op:Properties %s/>\n" % nsdecls("op", "vt")
     _offset_pattern = re.compile("([+-])(\\d\\d):(\\d\\d)")
 
     @classmethod
     def new(cls):
         """
-        Return a new ``<property>`` element
+        Return a new ``<op:Properties>`` element
         """
         xml = cls._customProperties_tmpl
         custom_properties = parse_xml(xml)
diff --git a/src/docx/oxml/ns.py b/src/docx/oxml/ns.py
index f179f61fc..89afe8a37 100644
--- a/src/docx/oxml/ns.py
+++ b/src/docx/oxml/ns.py
@@ -8,7 +8,7 @@
     "a": "http://schemas.openxmlformats.org/drawingml/2006/main",
     "c": "http://schemas.openxmlformats.org/drawingml/2006/chart",
     "cp": "http://schemas.openxmlformats.org/package/2006/metadata/core-properties",
-    "cup": "http://schemas.openxmlformats.org/officeDocument/2006/custom-properties",
+    "op": "http://schemas.openxmlformats.org/officeDocument/2006/custom-properties",
     "dc": "http://purl.org/dc/elements/1.1/",
     "dcmitype": "http://purl.org/dc/dcmitype/",
     "dcterms": "http://purl.org/dc/terms/",