diff --git a/tox.ini b/tox.ini
index 0c2ba060b1..ef543890f9 100644
--- a/tox.ini
+++ b/tox.ini
@@ -94,6 +94,7 @@ commands =
     # Add this so that coverage will run on subprocesses
     /bin/sh -c 'echo "import coverage; coverage.process_startup()" > {envsitepackagesdir}/../sitecustomize.py'
 
+    # Install Synapse itself. This won't update any libraries.
     pip install -e .
 
     {envbindir}/coverage run "{envbindir}/trial"  {env:TRIAL_FLAGS:} {posargs:tests} {env:TOXSUFFIX:}