diff --git a/Lib/sysconfig.py b/Lib/sysconfig.py
index eccd304..4f0a9a9 100644
--- a/Lib/sysconfig.py
+++ b/Lib/sysconfig.py
@@ -139,8 +139,8 @@
     _PROJECT_BASE = _safe_realpath(os.path.join(_PROJECT_BASE, pardir, pardir))
 
 # set for cross builds
-if "_PROJECT_BASE" in os.environ:
-    _PROJECT_BASE = _safe_realpath(os.environ["_PROJECT_BASE"])
+if "_PYTHON_PROJECT_BASE" in os.environ:
+    _PROJECT_BASE = _safe_realpath(os.environ["_PYTHON_PROJECT_BASE"])
 
 def _is_python_source_dir(d):
     for fn in ("Setup.dist", "Setup.local"):