diff --git a/modules/launch_utils.py b/modules/launch_utils.py
index 609a181e0..231ebc5f5 100644
--- a/modules/launch_utils.py
+++ b/modules/launch_utils.py
@@ -190,7 +190,7 @@ def run_extension_installer(extension_dir):
 
     try:
         env = os.environ.copy()
-        env['PYTHONPATH'] = os.path.abspath(".")
+        env['PYTHONPATH'] = f"{os.path.abspath('.')}{os.pathsep}{env['PYTHONPATH']}"
 
         print(run(f'"{python}" "{path_installer}"', errdesc=f"Error running install.py for extension {extension_dir}", custom_env=env))
     except Exception as e: