diff --git a/conformance-test.sh b/conformance-test.sh
index a1560a757..8b1e82f5b 100755
--- a/conformance-test.sh
+++ b/conformance-test.sh
@@ -76,7 +76,8 @@ else
 	venv "${TMP_DIR}/cwl-conformance-venv"
 	pip install -U setuptools wheel pip
 	pip uninstall -y cwltool
-	pip install "${SCRIPT_DIRECTORY}" -r"${SCRIPT_DIRECTORY}/requirements.txt"
+	pip install -r"${SCRIPT_DIRECTORY}/mypy-requirements.txt"
+	CWLTOOL_USE_MYPYC=1 MYPYPATH="${SCRIPT_DIRECTORY}/mypy-stubs" pip install "${SCRIPT_DIRECTORY}" -r"${SCRIPT_DIRECTORY}/requirements.txt"
 	pip install 'cwltest>=2.3' pytest-cov pytest-xdist
 fi