diff --git a/docs/source/debugging.rst b/docs/source/debugging.rst
index 56c96607bc..19c75acae1 100644
--- a/docs/source/debugging.rst
+++ b/docs/source/debugging.rst
@@ -1,8 +1,8 @@
 .. testsetup:: *
 
     from pytorch_lightning.trainer.trainer import Trainer
-    
- .. _debugging:
+
+.. _debugging:
 
 Debugging
 =========
diff --git a/pytorch_lightning/accelerators/ddp_backend.py b/pytorch_lightning/accelerators/ddp_backend.py
index 62e8344c17..2ecf084774 100644
--- a/pytorch_lightning/accelerators/ddp_backend.py
+++ b/pytorch_lightning/accelerators/ddp_backend.py
@@ -87,7 +87,7 @@ class DDPBackend(object):
         command = [sys.executable] + command
 
         # since this script sets the visible devices we replace the gpus flag with a number
-        num_gpus = os.environ['CUDA_VISIBLE_DEVICES'].split(',').__len__()
+        num_gpus = os.environ.get('CUDA_VISIBLE_DEVICES', []).split(',').__len__()
 
         if '--gpus' in command:
             gpu_flag_idx = command.index('--gpus')