diff --git a/tox.ini b/tox.ini
index 6328399c0f68..4d79799c6e02 100644
--- a/tox.ini
+++ b/tox.ini
@@ -21,7 +21,7 @@ deps = -r{toxinidir}/test-requirements.txt
 commands =
   py27: bash tools/pretty_tox.sh '{posargs}'
   py{34,35}: bash tools/pretty_tox3.sh '{posargs}'
-passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY OS_DEBUG
+passenv = http_proxy HTTP_PROXY https_proxy HTTPS_PROXY no_proxy NO_PROXY OS_DEBUG GENERATE_HASHES
 # there is also secret magic in pretty_tox.sh which lets you run in a fail only
 # mode. To do this define the TRACE_FAILONLY environmental variable.