diff --git a/noxfile.py b/noxfile.py
index 58ec712..4474a1d 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -6,7 +6,9 @@
 
 @nox.session(python=py_versions, reuse_venv=True)
 def blacken(session):
-    session.install("black")
+    # pinning black version until unstable formatting is fixed
+    # https://github.com/psf/black/issues/1629
+    session.install("black==19.10b0")
     session.run("black", "fables", "tests", "noxfile.py", "setup.py")