diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index a1c4d626..7cc595b1 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: ["3.10"]
+        python-version: ["3.9", "3.10"]
       max-parallel: 5
 
     steps:
diff --git a/build-env.yml b/build-env.yml
index 8091a163..35a00bce 100644
--- a/build-env.yml
+++ b/build-env.yml
@@ -14,7 +14,7 @@ dependencies:
   - numpy
   - pip
   - pytest-cov
-  - python==3.10.14
+  - python==3.9
   - scipy
   - snakemake==7.1.1
   - pip:
diff --git a/dadi_cli/BestFit.py b/dadi_cli/BestFit.py
index 28b4a901..0499baae 100644
--- a/dadi_cli/BestFit.py
+++ b/dadi_cli/BestFit.py
@@ -1,5 +1,6 @@
 import glob, sys
 import numpy as np
+from typing import Optional
 from dadi_cli.Models import get_model
 from dadi_cli.Pdfs import get_dadi_pdf_params
 
@@ -12,7 +13,7 @@ def get_bestfit_params(
     delta: float, 
     Nclose: int = 3, 
     Nbest: int = 100
-) -> (np.array | None):
+) -> Optional[np.array]:
     """
     Obtains best-fit parameters from optimization results, filters them based on log-likelihood
     differences, boundaries, and ranks them, finally outputs the results to a specified file.
diff --git a/mkdocs-env.yml b/mkdocs-env.yml
index 849d2bf6..ea96030f 100644
--- a/mkdocs-env.yml
+++ b/mkdocs-env.yml
@@ -18,7 +18,7 @@ dependencies:
   - numpy
   - pip
   - pytest-cov
-  - python==3.10.14
+  - python==3.9
   - python-markdown-math
   - scipy
   - dadi