diff --git a/publishconf.py b/publishconf.py
index 9e297520..16051b55 100644
--- a/publishconf.py
+++ b/publishconf.py
@@ -10,8 +10,9 @@
 from pelicanconf import *
 
 
-if "READTHEDOCS" in os.environ:
-    # Make all URLs "domainless" in the RTD preview
+if os.environ.get("READTHEDOCS_VERSION_TYPE") == "external":
+    # Make all URLs "domainless" in the RTD PR preview
+    # https://docs.readthedocs.io/en/stable/reference/environment-variables.html
     SITEURL = ""
 else:
     # This setting is needed to make the RSS/Atom feeds generate correctly