diff --git a/src/core/thread/indirect_sender.cpp b/src/core/thread/indirect_sender.cpp
index f2a71c4a7..461dff1d4 100644
--- a/src/core/thread/indirect_sender.cpp
+++ b/src/core/thread/indirect_sender.cpp
@@ -84,15 +84,14 @@ exit:
     mEnabled = false;
 }
 
-otError IndirectSender::AddMessageForSleepyChild(Message &aMessage, Child &aChild)
+void IndirectSender::AddMessageForSleepyChild(Message &aMessage, Child &aChild)
 {
-    otError  error = OT_ERROR_NONE;
     uint16_t childIndex;
 
-    VerifyOrExit(!aChild.IsRxOnWhenIdle(), error = OT_ERROR_INVALID_STATE);
+    OT_ASSERT(!aChild.IsRxOnWhenIdle());
 
     childIndex = Get<ChildTable>().GetChildIndex(aChild);
-    VerifyOrExit(!aMessage.GetChildMask(childIndex), error = OT_ERROR_ALREADY);
+    VerifyOrExit(!aMessage.GetChildMask(childIndex), OT_NOOP);
 
     aMessage.SetChildMask(childIndex);
     mSourceMatchController.IncrementMessageCount(aChild);
@@ -100,7 +99,7 @@ otError IndirectSender::AddMessageForSleepyChild(Message &aMessage, Child &aChil
     RequestMessageUpdate(aChild);
 
 exit:
-    return error;
+    return;
 }
 
 otError IndirectSender::RemoveMessageFromSleepyChild(Message &aMessage, Child &aChild)
diff --git a/src/core/thread/indirect_sender.hpp b/src/core/thread/indirect_sender.hpp
index 8d081b1eb..98e2c53df 100644
--- a/src/core/thread/indirect_sender.hpp
+++ b/src/core/thread/indirect_sender.hpp
@@ -152,12 +152,8 @@ public:
      * @param[in] aMessage  The message to add.
      * @param[in] aChild    The (sleepy) child for indirect transmission.
      *
-     * @retval OT_ERROR_NONE           Successfully added the message for indirect transmission.
-     * @retval OT_ERROR_ALREADY        The message was already added for indirect transmission to same child.
-     * @retval OT_ERROR_INVALID_STATE  The child is not sleepy.
-     *
      */
-    otError AddMessageForSleepyChild(Message &aMessage, Child &aChild);
+    void AddMessageForSleepyChild(Message &aMessage, Child &aChild);
 
     /**
      * This method removes a message for indirect transmission to a sleepy child.
diff --git a/src/core/thread/mesh_forwarder_ftd.cpp b/src/core/thread/mesh_forwarder_ftd.cpp
index f115e717f..1c6ac1a45 100644
--- a/src/core/thread/mesh_forwarder_ftd.cpp
+++ b/src/core/thread/mesh_forwarder_ftd.cpp
@@ -87,7 +87,7 @@ otError MeshForwarder::SendMessage(Message &aMessage)
 
                         if (!child.IsRxOnWhenIdle())
                         {
-                            IgnoreError(mIndirectSender.AddMessageForSleepyChild(aMessage, child));
+                            mIndirectSender.AddMessageForSleepyChild(aMessage, child);
                         }
                     }
                 }
@@ -101,7 +101,7 @@ otError MeshForwarder::SendMessage(Message &aMessage)
 
                         if (mle.IsSleepyChildSubscribed(ip6Header.GetDestination(), child))
                         {
-                            IgnoreError(mIndirectSender.AddMessageForSleepyChild(aMessage, child));
+                            mIndirectSender.AddMessageForSleepyChild(aMessage, child);
                         }
                     }
                 }
@@ -112,7 +112,7 @@ otError MeshForwarder::SendMessage(Message &aMessage)
         {
             // destined for a sleepy child
             Child &child = *static_cast<Child *>(neighbor);
-            IgnoreError(mIndirectSender.AddMessageForSleepyChild(aMessage, child));
+            mIndirectSender.AddMessageForSleepyChild(aMessage, child);
         }
         else
         {
@@ -127,7 +127,7 @@ otError MeshForwarder::SendMessage(Message &aMessage)
     {
         Child *child = Get<Utils::ChildSupervisor>().GetDestination(aMessage);
         OT_ASSERT((child != NULL) && !child->IsRxOnWhenIdle());
-        IgnoreError(mIndirectSender.AddMessageForSleepyChild(aMessage, *child));
+        mIndirectSender.AddMessageForSleepyChild(aMessage, *child);
         break;
     }