diff --git a/packages/td-tools/src/td-parser.ts b/packages/td-tools/src/td-parser.ts
index 6c9d2d2f7..0c19a2992 100644
--- a/packages/td-tools/src/td-parser.ts
+++ b/packages/td-tools/src/td-parser.ts
@@ -241,9 +241,9 @@ export function serializeTD(thing: Thing): string {
     } else if (copy.properties != null) {
         // add mandatory fields (if missing): observable, writeOnly, and readOnly
         for (const prop of Object.values(copy.properties)) {
-            prop.readOnly ??= false;
-            prop.writeOnly ??= false;
-            prop.observable ??= false;
+            adjustBooleanField(prop, "readOnly")
+            adjustBooleanField(prop, "writeOnly")
+            adjustBooleanField(prop, "observable")
         }
     }