diff --git a/packages/vats/src/core/chain-behaviors.js b/packages/vats/src/core/chain-behaviors.js
index 18dca0c1c4a..15e5a11b2c1 100644
--- a/packages/vats/src/core/chain-behaviors.js
+++ b/packages/vats/src/core/chain-behaviors.js
@@ -80,7 +80,9 @@ export const bridgeCoreEval = async allPowers => {
                 });
 
                 // Evaluate the code in the context of the globals.
-                const behavior = new Compartment(globals).evaluate(code);
+                const compartment = new Compartment(globals);
+                harden(compartment.globalThis);
+                const behavior = compartment.evaluate(code);
                 return behavior(powers);
               }),
             ),