diff --git a/src/libs/languageserverprotocol/jsonrpcmessages.h b/src/libs/languageserverprotocol/jsonrpcmessages.h
index ebf51b4c14c..c33e46422ca 100644
--- a/src/libs/languageserverprotocol/jsonrpcmessages.h
+++ b/src/libs/languageserverprotocol/jsonrpcmessages.h
@@ -290,7 +290,10 @@ public:
         return val.isUndefined() ? std::nullopt : std::make_optional(fromJsonValue<Error>(val));
     }
     void setError(const Error &error)
-    { m_jsonObject.insert(errorKey, QJsonValue(error)); }
+    {
+        QTC_CHECK(error.isValid());
+        m_jsonObject.insert(errorKey, QJsonValue(error));
+    }
     void clearError() { m_jsonObject.remove(errorKey); }
 
     bool isValid(QString *errorMessage) const override
diff --git a/src/plugins/languageclient/client.cpp b/src/plugins/languageclient/client.cpp
index 5c99329675a..aca6ab50098 100644
--- a/src/plugins/languageclient/client.cpp
+++ b/src/plugins/languageclient/client.cpp
@@ -1891,6 +1891,7 @@ void ClientPrivate::handleMethod(const QString &method, const MessageId &id, con
         Response<JsonObject, JsonObject> response(id);
         ResponseError<JsonObject> error;
         error.setCode(ResponseError<JsonObject>::MethodNotFound);
+        error.setMessage(QString("The client cannot handle the method '%1'.").arg(method));
         response.setError(error);
         sendResponse(response);
     }