diff --git a/source/optims/constantFolding.ts b/source/optims/constantFolding.ts
index 5f6cfcc..41cf150 100644
--- a/source/optims/constantFolding.ts
+++ b/source/optims/constantFolding.ts
@@ -159,7 +159,8 @@ function isFoldable(
   rule: RuleNode | undefined,
   contextRules: Set<RuleName>,
 ): boolean {
-  if (!rule) {
+  // We need to keep empty rules as they are used as namespace.
+  if (!rule || isEmptyRule(rule)) {
     return false
   }