diff --git a/hooks/kroki.py b/hooks/kroki.py
index 54aa5ed..39a2629 100644
--- a/hooks/kroki.py
+++ b/hooks/kroki.py
@@ -3,7 +3,7 @@ import zlib
 from mkdocs.utils.cache import download_and_cache_url
 from datetime import timedelta
 from mkdocs.plugins import get_plugin_logger
-from urllib.request import build_opener, install_opener
+from urllib.request import build_opener, install_opener, _opener
 import hashlib
 from functools import partial
 from mkdocs.utils import write_file
@@ -51,11 +51,6 @@ def fence_div_format(site_dir, source, language, *args, attrs, **kwargs):
     return f"<img src='{image_url}' alt='{title}' />"
 
 def on_config(config):
-    # Override user-agent so kroki accepts it
-    opener = build_opener()
-    opener.addheaders = [('User-Agent','mkdocs')]
-    install_opener(opener)
-
     custom_fences = [
         {
             "name": diagram,
@@ -69,3 +64,14 @@ def on_config(config):
         "custom_fences": custom_fences
     }
     return config
+
+
+def on_startup(command, dirty):
+    # Override user-agent so kroki accepts it
+    if _opener is None:
+        opener = build_opener()
+        install_opener(opener)
+    else:
+        opener = _opener
+
+    opener.addheaders = [('User-Agent','mkdocs')]