diff --git a/packages/jsii-reflect/lib/enum.ts b/packages/jsii-reflect/lib/enum.ts
index ac9c6bbd64..6e4db8eaa0 100644
--- a/packages/jsii-reflect/lib/enum.ts
+++ b/packages/jsii-reflect/lib/enum.ts
@@ -18,7 +18,7 @@ export class EnumType extends Type {
     return this.spec.members.map((m) => new EnumMember(this, m));
   }
 
-  public isEnumType() {
+  public isEnumType(): this is EnumType {
     return true;
   }
 }
diff --git a/packages/jsii-reflect/lib/interface.ts b/packages/jsii-reflect/lib/interface.ts
index d02ec2933b..05567cce07 100644
--- a/packages/jsii-reflect/lib/interface.ts
+++ b/packages/jsii-reflect/lib/interface.ts
@@ -72,11 +72,11 @@ export class InterfaceType extends ReferenceType {
     return Object.fromEntries(this._getMethods(inherited, this));
   }
 
-  public isDataType() {
+  public isDataType(): this is InterfaceType {
     return !!this.spec.datatype;
   }
 
-  public isInterfaceType() {
+  public isInterfaceType(): this is InterfaceType {
     return true;
   }