diff --git a/src/core/thread/mle_router.cpp b/src/core/thread/mle_router.cpp
index 89a8b3e3d..c4dca1732 100644
--- a/src/core/thread/mle_router.cpp
+++ b/src/core/thread/mle_router.cpp
@@ -3065,7 +3065,7 @@ void MleRouter::SendChildUpdateResponse(Child *                 aChild,
             break;
 
         case Tlv::kResponse:
-            SuccessOrExit(error = AppendResponse(*message, aChallenge.GetChallenge(), aChallenge.GetLength()));
+            SuccessOrExit(error = AppendResponse(*message, aChallenge.GetChallenge(), aChallenge.GetChallengeLength()));
             break;
 
         case Tlv::kSourceAddress: