diff --git a/src/CoAPNet/CoapBlockStream.cs b/src/CoAPNet/CoapBlockStream.cs
index 522337e..f79458a 100644
--- a/src/CoAPNet/CoapBlockStream.cs
+++ b/src/CoAPNet/CoapBlockStream.cs
@@ -102,12 +102,12 @@ public CoapBlockStream(CoapClient client, CoapMessage baseMessage, ICoapEndpoint
             _client = client ?? throw new ArgumentNullException(nameof(client));
             _endpoint = endpoint;
 
-            if (!_baseMessage.Code.IsRequest())
-                throw new InvalidOperationException($"Can not create a {nameof(CoapBlockStream)} with a {nameof(baseMessage)}.{nameof(baseMessage.Type)} of {baseMessage.Type}");
-
             _baseMessage = baseMessage?.Clone()
                            ?? throw new ArgumentNullException(nameof(baseMessage));
 
+            if (!_baseMessage.Code.IsRequest())
+                throw new InvalidOperationException($"Can not create a {nameof(CoapBlockStream)} with a {nameof(baseMessage)}.{nameof(baseMessage.Type)} of {baseMessage.Type}");
+
             _writerTask = WriteBlocksAsync();