--- a/layout/tools/reftest/reftest.jsm
+++ b/layout/tools/reftest/reftest.jsm
@@ -709,21 +709,22 @@ function BuildConditionSandbox(aURL) {
 #endif
 
 #if MOZ_WEBRTC
     sandbox.webrtc = true;
 #else
     sandbox.webrtc = false;
 #endif
 
-sandbox.stylo =
 #ifdef MOZ_STYLO
-    prefs.getBoolPref("layout.css.servo.enabled", false);
+    sandbox.stylo = prefs.getBoolPref("layout.css.servo.enabled", false);
+    sandbox.styloVsGecko = gCompareStyloToGecko;
 #else
-    false;
+    sandbox.stylo = false;
+    sandbox.styloVsGecko = false;
 #endif
 
 #ifdef RELEASE_OR_BETA
     sandbox.release_or_beta = true;
 #else
     sandbox.release_or_beta = false;
 #endif