diff --git a/src/core/coap/coap.hpp b/src/core/coap/coap.hpp
index 1d40b0dd0..72f1afc8b 100644
--- a/src/core/coap/coap.hpp
+++ b/src/core/coap/coap.hpp
@@ -304,7 +304,9 @@ public:
      */
     static void RemoveFrom(Message &aMessage)
     {
-        assert(aMessage.SetLength(aMessage.GetLength() - sizeof(EnqueuedResponseHeader)) == OT_ERROR_NONE);
+        otError error = aMessage.SetLength(aMessage.GetLength() - sizeof(EnqueuedResponseHeader));
+        assert(error == OT_ERROR_NONE);
+        OT_UNUSED_VARIABLE(error);
     }
 
     /**