diff --git a/test_requirements.txt b/test_requirements.txt
index e52336266..30d0a2a19 100644
--- a/test_requirements.txt
+++ b/test_requirements.txt
@@ -1,4 +1,4 @@
-black>=20.8b0
+black==21.12b0
 check-manifest
 flake8
 isort
diff --git a/tests/test_welltest_dpds.py b/tests/test_welltest_dpds.py
index 634932407..73f299e8c 100644
--- a/tests/test_welltest_dpds.py
+++ b/tests/test_welltest_dpds.py
@@ -265,4 +265,4 @@ def test_to_csv(tmp_path):
 
     dframe = pd.read_csv("mock.csv", skipinitialspace=True)
     assert dframe["vec"].size == 4
-    assert pytest.approx(dframe["vecb"].iloc[-1] == 100)
+    assert dframe["vecb"].iloc[-1] == pytest.approx(100)