diff --git a/tools/build-requirements.txt b/tools/build-requirements.txt
index c01d5e289f..5aef83b54c 100644
--- a/tools/build-requirements.txt
+++ b/tools/build-requirements.txt
@@ -3,4 +3,4 @@
 # environment to detect when to re-create the virtualenv so the hash
 # needs to change whenever a package is updated.
 
-git+https://github.com/igraph/stimulus@0.21.2#egg=stimulus
+git+https://github.com/igraph/stimulus@0.21.4#egg=stimulus