diff --git a/docs/requirements_rtd.txt b/docs/requirements_rtd.txt
index db0f055..aa97860 100644
--- a/docs/requirements_rtd.txt
+++ b/docs/requirements_rtd.txt
@@ -1,6 +1,6 @@
 setuptools>=41.0.1
 matplotlib>=3.2
-sphinx>=3.0.0
+sphinx>=3,<7
 sphinx_autodoc_typehints>=1.11.0
 recommonmark>=0.6.0
 sphinx_rtd_theme>=0.5.0