diff --git a/pyproject.toml b/pyproject.toml
index a84e37bba7f..7b4ac0310b7 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -53,6 +53,12 @@
 
 [tool.black]
 target-version = ['py35', 'py36', 'py37', 'py38']
+exclude = '''
+/(
+    # Work around https://github.com/psf/black/issues/1629 for this file
+    src/twisted/words/test/test_irc.py
+)/
+'''
 
 [tool.isort]
 line_length = 88