diff --git a/docs/advanced.md b/docs/advanced.md
index 4438cb2d..5f900cf6 100644
--- a/docs/advanced.md
+++ b/docs/advanced.md
@@ -1100,7 +1100,7 @@ def handler(request):
 
 
 # Switch to a mock transport, if the TESTING environment variable is set.
-if os.environ['TESTING'].upper() == "TRUE":
+if os.environ.get('TESTING', '').upper() == "TRUE":
     transport = httpx.MockTransport(handler)
 else:
     transport = httpx.HTTPTransport()