diff --git a/documentation/conf.py b/documentation/conf.py
index 3f867e0a3a..c86a145f9d 100644
--- a/documentation/conf.py
+++ b/documentation/conf.py
@@ -215,7 +215,7 @@ def install_doxygen():
 nbsphinx_prolog = (
     f"{{% set {ref=} %}}"
     r"""
-    {% set docname = "documentation/" + env.doc2path(env.docname, base=False) %}
+    {% set docname = "documentation/" + env.doc2path(env.docname, base=False)|string %}
     .. raw:: html
 
         <div class="note">
diff --git a/documentation/rtd_requirements.txt b/documentation/rtd_requirements.txt
index 06cf506666..05e04bc957 100644
--- a/documentation/rtd_requirements.txt
+++ b/documentation/rtd_requirements.txt
@@ -1,5 +1,5 @@
 # NOTE: relative paths are expected to be relative to the repository root
-sphinx
+sphinx<8
 mock>=5.0.2
 setuptools>=67.7.2
 pysb>=1.11.0