diff --git a/requirements_builder.ABOUT b/requirements_builder.ABOUT
index 729b8d1..5407b2b 100644
--- a/requirements_builder.ABOUT
+++ b/requirements_builder.ABOUT
@@ -87,6 +87,6 @@ testing =
     pycodestyle
 
 docs =
-    Sphinx >= 3.3.1
+    Sphinx >= 3.3.1,<7.0.0
     sphinx-rtd-theme >= 0.5.0
     doc8 >= 0.8.1
diff --git a/setup.cfg b/setup.cfg
index 0d8b679..ec86a67 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -88,6 +88,6 @@ testing =
     pytest-rerunfailures
 
 docs =
-    Sphinx >= 3.3.1
+    Sphinx >= 3.3.1,<7.0.0
     sphinx-rtd-theme >= 0.5.0
     doc8 >= 0.8.1