diff --git a/repo2docker/app.py b/repo2docker/app.py
index 5ac6b953..f809480e 100755
--- a/repo2docker/app.py
+++ b/repo2docker/app.py
@@ -431,7 +431,7 @@ class Repo2Docker(Application):
         help="""
         Base image to use when building docker images.
 
-        Should be an ubuntu derivative, minimum 18.04.
+        Should be an Ubuntu derivative, minimum 18.04.
         """,
     )