diff --git a/rtd_requirements.txt b/rtd_requirements.txt
index e2e69f245..bb1e75491 100644
--- a/rtd_requirements.txt
+++ b/rtd_requirements.txt
@@ -1,3 +1,4 @@
+funcparserlib==1.0.0.a0   # required by: https://github.com/vlasovskikh/funcparserlib/issues/70
 sphinxcontrib-blockdiag>=1.5.0
 sphinx-rtd-theme>=0.2.4
 apispec[yaml]==3.3.0