diff --git a/doc/requirements.txt b/doc/requirements.txt
index 1ff1bd640..b93bdc68b 100644
--- a/doc/requirements.txt
+++ b/doc/requirements.txt
@@ -1,3 +1,4 @@
 zuul-sphinx>=0.1.1
 sphinxcontrib-blockdiag>=1.1.0
+funcparserlib>=1.0.0a0  # https://github.com/blockdiag/blockdiag/pull/148
 sphinxcontrib-seqdiag