diff --git a/lib/carto/torque-reference.js b/lib/carto/torque-reference.js
index 0b9f204..65cb946 100644
--- a/lib/carto/torque-reference.js
+++ b/lib/carto/torque-reference.js
@@ -311,7 +311,8 @@ var _mapnik_reference_latest = {
                 "type": "float",
                 "doc": "The opacity of the polygon",
                 "default-value": 1,
-                "default-meaning": "opaque"
+                "default-meaning": "opaque",
+                "expression": true
             },
             "gamma": {
                 "css": "polygon-gamma",