diff --git a/repo2docker/buildpacks/pipfile/__init__.py b/repo2docker/buildpacks/pipfile/__init__.py
index 7b9e3386..fc922c27 100644
--- a/repo2docker/buildpacks/pipfile/__init__.py
+++ b/repo2docker/buildpacks/pipfile/__init__.py
@@ -105,7 +105,9 @@ class PipfileBuildPack(CondaBuildPack):
         working_directory = self.binder_dir or "."
 
         # install pipenv to install dependencies within Pipfile.lock or Pipfile
-        assemble_scripts.append(("${NB_USER}", "${KERNEL_PYTHON_PREFIX}/pip install pipenv=2018.11.26"))
+        assemble_scripts.append(
+            ("${NB_USER}", "${KERNEL_PYTHON_PREFIX}/pip install pipenv=2018.11.26")
+        )
 
         # NOTES:
         # - Without prioritizing the PATH to KERNEL_PYTHON_PREFIX over