diff --git a/docs/requirements.txt b/docs/requirements.txt
index d68cecc8..c7725489 100644
--- a/docs/requirements.txt
+++ b/docs/requirements.txt
@@ -18,3 +18,4 @@ fgspectra>=1.1.0
 syslibrary
 sphinx>=4.0
 sphinx_rtd_theme
+typing_extensions; python_version == '3.9'
diff --git a/pyproject.toml b/pyproject.toml
index 8d2d9fda..7089cf1a 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -23,7 +23,8 @@ dependencies = [
     "camb",
     "getdist",
     "syslibrary>=0.2.0",
-    "mflike>=1.0.0"
+    "mflike>=1.0.0",
+    "typing_extensions; python_version == '3.9'",
 ]
 
 [project.urls]
diff --git a/requirements.txt b/requirements.txt
index 82b65be2..d51971e7 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -19,3 +19,4 @@ sacc
 fgspectra>=1.1.0
 syslibrary
 pyhalomodel
+typing_extensions; python_version == '3.9'