diff --git a/.travis.yml b/.travis.yml
index 57bd03c6..18dca46c 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -7,6 +7,11 @@ matrix:
     - python: 3.7
       env: PUBLISH_DOCS=1
     - python: 3.7
+      # We still test against jsonschema 2.x because pip's dependency resolver
+      # very often gives us 2.x even though we ask for 3.x. The event-model
+      # code has codepaths for either, conditional on what is installed.
+      # See https://github.com/bluesky/event-model/pull/175 and
+      # https://github.com/bluesky/event-model/issues/145 for more context.
       env: JSONSCHEMA_2=1
     - python: 3.8
     - python: nightly