diff --git a/src/black/__init__.py b/src/black/__init__.py
index ebc7ac8eda5..8ab5b47f974 100644
--- a/src/black/__init__.py
+++ b/src/black/__init__.py
@@ -279,7 +279,7 @@ def validate_regex(
     is_flag=True,
     help=(
         "Format all input files like Jupyter Notebooks regardless of file extension."
-        "This is useful when piping source on standard input."
+        " This is useful when piping source on standard input."
     ),
 )
 @click.option(