diff --git a/modules/launch_utils.py b/modules/launch_utils.py
index 93c8c8ca7..e1c9cfbec 100644
--- a/modules/launch_utils.py
+++ b/modules/launch_utils.py
@@ -196,7 +196,7 @@ def run_extension_installer(extension_dir):
 
     try:
         env = os.environ.copy()
-        env['PYTHONPATH'] = f"{os.path.abspath('.')}{os.pathsep}{env['PYTHONPATH']}"
+        env['PYTHONPATH'] = f"{os.path.abspath('.')}{os.pathsep}{env.get('PYTHONPATH', '')}"
 
         print(run(f'"{python}" "{path_installer}"', errdesc=f"Error running install.py for extension {extension_dir}", custom_env=env))
     except Exception as e: