diff --git a/src/core/thread/mle_router.cpp b/src/core/thread/mle_router.cpp
index 0bafc4807..69fe6a63d 100644
--- a/src/core/thread/mle_router.cpp
+++ b/src/core/thread/mle_router.cpp
@@ -4506,7 +4506,7 @@ otError MleRouter::AppendChildAddresses(Message &aMessage, Child &aChild)
     AddressRegistrationEntry entry;
     Lowpan::Context context;
     uint8_t length = 0;
-    uint8_t startOffset = static_cast<uint8_t>(aMessage.GetLength());
+    uint16_t startOffset = aMessage.GetLength();
 
     tlv.SetType(Tlv::kAddressRegistration);
     SuccessOrExit(error = aMessage.Append(&tlv, sizeof(tlv)));