diff --git a/packages/aws-cdk-lib/aws-iam/lib/principals.ts b/packages/aws-cdk-lib/aws-iam/lib/principals.ts
index 6cce789ca1fe0..a5ab8834333dc 100644
--- a/packages/aws-cdk-lib/aws-iam/lib/principals.ts
+++ b/packages/aws-cdk-lib/aws-iam/lib/principals.ts
@@ -53,7 +53,7 @@ export interface IPrincipal extends IGrantable {
    * Can be a Token - in that case,
    * it's assumed to be AWS::AccountId.
    */
-  readonly principalAccount?: string;
+  readonly principalAccount?: string | undefined;
 
   /**
    * Add to the policy of this principal.