diff --git a/docxcompose/properties.py b/docxcompose/properties.py
index aa8cd2d..51f2ac0 100644
--- a/docxcompose/properties.py
+++ b/docxcompose/properties.py
@@ -15,7 +15,7 @@
 from lxml.etree import QName
 from six import binary_type
 from six import text_type
-import pkg_resources
+import importlib.resources as importlib_resources
 import re
 
 
@@ -108,8 +108,8 @@ def __init__(self, doc):
             self._element = parse_xml(part.blob)
 
     def _part_template(self):
-        return pkg_resources.resource_string(
-            'docxcompose', 'templates/custom.xml')
+        ref = importlib_resources.files('docxcompose').joinpath('templates/custom.xml')
+        return ref.read_bytes()
 
     def _update_part(self):
         if self.part is None: