diff --git a/src/core/uid-manager.h b/src/core/uid-manager.h
index 5cc6c1adc..e4648e31a 100644
--- a/src/core/uid-manager.h
+++ b/src/core/uid-manager.h
@@ -27,6 +27,9 @@
 
 namespace ns3 {
 
+/**
+ * zero is never a valid uid value.
+ */
 class UidManager
 {
 public: