diff --git a/requirements.txt b/requirements.txt
index 6b570fa..7e5d4c6 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,17 +1,19 @@
 # Add here external Python modules dependencies, one per line. Module names
 # should match https://pypi.python.org/pypi names. For the full spec or
 # dependency lines, see https://pip.readthedocs.org/en/1.1/requirements.html
 
 arrow
 celery >= 4
+# Not a direct dependency: missing from celery 4.4.4
+future >= 0.18.0
 Click
 elasticsearch > 5.4
 flask
 pika >= 1.1.0
 psycopg2
 pyyaml
 vcversioner
 setuptools
 
 # test dependencies
 # hypothesis