export const assertInterfaceGuard = specimen => {
   mustMatch(specimen, InterfaceGuardShape, 'interfaceGuard');
 };
 harden(assertInterfaceGuard);
+
+const { comparator: passableComparator } = makeComparatorKit();
 
 /**
  * @param {string} interfaceName
  * @param {Record<string, MethodGuard>} methodGuards
  * @param {{sloppy?: boolean}} [options]
  * @returns {InterfaceGuard}
  */
 const makeInterfaceGuard = (interfaceName, methodGuards, options = {}) => {
   const { sloppy = false } = options;
+  const stringMethodGuards = {};
+  const symbolMethodGuardsEntries = [];
+  for (const key of ownKeys(methodGuards)) {
+    const value = methodGuards[key];
+    if (typeof key === 'symbol') {
+      symbolMethodGuardsEntries.push([key, value]);
+    } else {
+      stringMethodGuards[key] = value;
+    }
+  }
+  // Ensure a consistent order.
+  symbolMethodGuardsEntries.sort(([k1], [k2]) => passableComparator(k1, k2));
   /** @type {InterfaceGuard} */
   const result = harden({
     klass: 'Interface',
     interfaceName,
-    methodGuards,
+    methodGuards: stringMethodGuards,
+    symbolMethodGuards: symbolMethodGuardsEntries,
     sloppy,
   });
   assertInterfaceGuard(result);
   return result;
 };